JP3291014B2 - Method and apparatus for producing ceramic teeth - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for manufacturing porcelain for prosthesis by baking porcelain on a crown such as a metal denture.

[0002]

2. Description of the Related Art In the field of dental treatment, in order to prosthetic a part of a tooth which has been lost due to occupation or the like, conventionally, a gold alloy crown is fixedly attached to a remaining root of a living tooth with a cap, and the tooth is fixed to the tooth. In recent years, gold alloys have been expensive, but in recent years, Co-Cr alloys or Ni-Cr alloys have been used, and the surface color of the gold alloy is approximated to the appearance of natural teeth to provide aesthetics. For this purpose, a method of baking an oxide-based porcelain to make the surface porcelain has been widely used. Recently,
Titanium metal or titanium alloy, which is less harmful to the human body than these alloys, has been awarded due to its excellent strength and toughness as well as corrosion resistance.

[0003] The manufacturing process of a ceramic tooth is as follows, taking a ceramic tooth having a denture body made of titanium as an example. (1) Titanium melted by vacuum arc melting is cast into a mold gap having a tooth profile formed by taking an impression from a remaining tooth to be covered with a crown and shaping to form a denture body.

(2) In order to make the titanium metal surface of the denture body opaque, opaque treatment is performed to apply a special porcelain and form a thin fired underlayer. (3) firing and applying a porcelain slurry on the firing base layer;
A body treatment for forming a transparent or translucent milky white layer is performed.
Next, the shape is corrected by grinding, and final dimensional adjustment is performed. (4) A stain process is performed in which a special porcelain (stain material) is applied and fired in order to apply a desired color close to the enamel of human teeth. (5) Glazing treatment by baking is performed to smooth the surface after the stain treatment by melting.

[0005] In the calcination consisting of the above four treatment steps, calcination of the opaque treatment is carried out at the highest temperature, and conventionally a calcination temperature of about 950 ° C was required. In recent years, low-melting porcelain has been developed, and the firing temperature is 750-800.
° C.

[0006]

In the sintering in each of the conventional sintering steps, the porcelain is fired at a high temperature using a sintering furnace in an air atmosphere capable of reducing pressure, and baked on a metal denture body surface. However, during this firing, by removing the bubbles contained in the porcelain to be softened and melted by maintaining the reduced pressure state,
If the porcelain is made more transparent and kept at high temperature after melting and degassing, the porcelain will hang down and the outer shape of the porcelain tooth will collapse. Was.

[0007] In such a method of firing ceramic teeth, when titanium or a titanium alloy is used as the metal material of the crown, the low fusible material generally has poor aesthetics, and also has a low porosity of 750 to 80.
Even at the sintering temperature of 0 ° C., there is a problem that an oxide layer is formed between the titanium metal and the porcelain on the surface during calcination and rapid cooling, and the porcelain cracks and separates in the oxide layer. Was.

In addition, an oxide film is formed on the titanium surface inside the ceramic tooth during the firing process, and even if an antioxidant is applied to the inner surface, the formation of the oxide film cannot be prevented. After firing, grinding the inner surface of the porcelain tooth to remove a slight oxide film increases the clearance between the surface of the remaining tooth and the inner surface of the porcelain tooth. There is also a problem that the occlusion is obstructed due to the deviation from the arrangement.

The above problem arises from the property that titanium metal or titanium alloy easily forms oxides or the like on the surface at high temperatures by combining with oxygen and nitrogen in the air. Therefore, in order to completely prevent oxidation during firing, it is necessary to insert the denture into the furnace, heat it while maintaining it in a high vacuum, and after firing, wait for the inside of the furnace to cool down and take it out. However, since the heat capacity in the furnace is large, a long time is required for cooling, and dripping of the molten porcelain at a high temperature cannot be avoided.

SUMMARY OF THE INVENTION In view of the above problems, the present invention aims to provide a method and apparatus for producing a ceramic tooth which prevents oxidation during the firing of the ceramic tooth, has excellent workability, and is inexpensive to manufacture. is there.

[0011]

SUMMARY OF THE INVENTION The method of the present invention for producing a porcelain tooth is a method for producing a porcelain tooth in which a porcelain coated on the surface of a denture body made of an easily oxidizable metal is baked in a heating furnace. Inserting the denture body coated with the porcelain teeth or porcelain material locked on the porcelain tooth holder into the heating furnace, and thereafter replacing the furnace atmosphere with a non-oxidizing atmosphere; and A baking step of reducing the non-oxidizing atmosphere, and heating the heating furnace to raise the temperature into the porcelain teeth, and then heating the porcelain teeth on the porcelain tooth holder in the high-temperature heating furnace; A cooling step of spraying a non-oxidizing gas to the ceramic tooth to rapidly cool the ceramic tooth to a temperature at which the denture body is not substantially oxidized in the atmosphere and to recover the pressure to the atmospheric pressure; A step of taking out the holding tool and the ceramic teeth.

[0012] The ceramic tooth manufacturing apparatus of the present invention comprises an airtight heating furnace main body connected to a gas introduction pipe for introducing a non-oxidizing gas and a vacuum pipe, and a refractory heat-resistant body inside the heating furnace main body. A heating chamber which is surrounded and in which a heating element is arranged;
The ceramic tooth holder which holds the ceramic teeth to be heated and baked in the heating chamber and is inserted and set in the heating chamber and the ceramic tooth holder which extends downward from the upper part of the heating furnace body into the heating chamber. A gas injection tube for injecting a non-oxidizing gas toward the held porcelain to rapidly cool the fired porcelain. In this apparatus, a furnace body opening communicating with the heating chamber is provided at a bottom of the heating furnace body, and the ceramic tooth holder is fixed on a substrate held on an elevating device. Wherein the baking table is inserted into the furnace through the furnace body opening, is disposed immediately below the pipe end of the gas injection tube, and the substrate is placed at the end of the furnace body opening. The one that comes into contact with the edge to seal the opening is preferably employed.

In the present invention, the easily oxidizable metal includes metal titanium, a titanium alloy, the above-mentioned Co—Cr alloy and Ni—Cr alloy.
Alloys and other metals and alloys that easily form a surface oxide layer at high temperatures are included.

[0014]

First, according to the present invention of the production method, the calcination is composed of four steps, the first substitution step, usually in the furnace preheating step,
Since the atmosphere in the furnace is replaced with a non-oxidizing gas, for example, an inert gas atmosphere, until the atmosphere is returned to an air atmosphere thereafter,
Even if the denture is made of titanium metal or titanium alloy, no surface oxidation occurs during heating.

In the second firing step, the porcelain is softened and melted by heating to become porcelain, and the inert gas atmosphere is reduced in pressure. Therefore, bubbles in the porcelain are easily removed to the reduced pressure atmosphere during softening and melting. Is done. If bubbles are contained in the porcelain of the surface layer of the porcelain teeth, the porcelain becomes cloudy and opaque due to the scattered light. However, the defoaming by the reduced pressure improves the transparency of the porcelain after cooling and makes the porcelain uniform.

In the third cooling step, a non-oxidizing gas, for example, an inert gas is blown onto the hot porcelain teeth having a uniform porcelain and defoamed to cool them. Even though the heat capacity in the furnace is large, the denture or ceramic teeth are small in shape and size and their heat capacity is small, so even when the furnace is at high temperature, the temperature drops rapidly by gas blowing, and the blowing gas is inert gas. Because it is a non-oxidizing gas, it is not oxidized during cooling and is therefore rapidly cooled to a temperature that does not oxidize when exposed to air. Further, the inside of the furnace may be simultaneously restored to the atmospheric pressure by blowing gas. In the fourth removal step, the porcelain tooth lowered to a temperature at which it is not oxidized in the air is removed from the furnace and allowed to cool.

The above-described method for producing a ceramic tooth is used for any of the above-described opaque treatment, body treatment, stain treatment, and glazing treatment. No oxide film is formed on the finished porcelain tooth, and no release layer due to the oxide layer is formed in the porcelain. Further, as the replacement gas and the injection gas in the furnace, nitrogen gas can be used instead of the inert gas. In this case, if the denture body is made of metal titanium or a titanium alloy, a nitride is formed on the surface, but the nitride is dense and stable and does not peel off.

Next, in the apparatus according to the present invention for carrying out the method for manufacturing a ceramic tooth, the heating furnace main body is airtight and a non-oxidizing gas introduction pipe and a vacuum pipe are connected. It is possible to use an active gas atmosphere or a nitrogen gas atmosphere or to reduce the pressure. The heating element is usually an electric heating element, but heats the ceramic tooth holder, especially the firing table, and the ceramic teeth placed and locked on the ceramic tooth holder, particularly the firing table, in the heating chamber to a predetermined temperature.

Since the gas injection pipe is provided with its pipe end opening facing the ceramic tooth held by the ceramic tooth holder, in the cooling step, an inert gas or nitrogen gas is blown to heat the high-temperature ceramic tooth. Cool rapidly. When the injection pipe has an expanded portion expanded on the opening side, and the straightening plate is mounted inside the expanded portion, the inert gas or the nitrogen gas flows between the peripheral edge of the straightening plate and the inner wall of the expanded portion. Since the jet flow spreads at intervals, the ceramic teeth placed on the ceramic tooth holder, particularly a plurality of firing teeth, are cooled substantially uniformly.

Even if the temperature in the heating chamber is high, the gas injection tube cools the porcelain tooth itself to a temperature at which the porcelain tooth itself is not oxidized in the air, so that the porcelain tooth can be immediately taken out of the furnace.
Therefore, the time required for firing the ceramic teeth can be reduced, and the process of firing the next ceramic teeth can be performed without cooling the inside of the furnace.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described first with reference to the drawings. FIG. 1 shows a cross-sectional view of a porcelain firing apparatus of the present invention. This apparatus is a vertical furnace (vertical furnace), and the upper part of a bottomed cylindrical furnace shell 10 is opened. The furnace lid 11 is air-tightly placed and fixed to the opening end, and the bottom 101 has an opening 102 at the center thereof.

Inside the furnace shell 10, a steel plate inner cylinder 12 is fixed. Inside the furnace shell 10, a hollow heat-resistant body 23 made of a refractory and a heat-generating body 5
And a transparent quartz tube 22 is disposed inside the quartz tube.
The space inside 2 is a heating chamber 2. In the heating element 5, in this example, a coil of a nichrome wire is vertically provided inside a vertical groove provided in the hollow wall of the heat-resistant body 23 in a circumferential direction.

The upper surface of the heat-resistant body 23 has a refractory block 24,
25, the heating element 5 and the upper part of the heating chamber 22 are covered. The straight pipe portion 42 of the argon gas injection pipe 4 is inserted into the small holes vertically penetrating the refractory blocks 24 and 25. Have been drooping.

As for the furnace bottom, the bottom 101 of the furnace shell 10 is used.
A firing table 30 made of a refractory is inserted inside the opening 102 and the quartz tube 22, and the substrate 31 of the firing table 30 has its peripheral upper surface airtightly contacting the bottom of the opening of the furnace shell bottom 101. It is held by an elevating device (not shown).

A heat-resistant small rod 32 is vertically provided on the upper surface of the firing table 30, and the ceramic teeth 3 to be fired are locked on the upper end of the small rod 32. At the time of charging, as shown in FIG.
The upper surface is held in the soaking area in the heating chamber 2 and is disposed immediately below the injection port 40 of the argon gas injection pipe 4.

As shown in FIG. 2, the argon gas injection pipe has a quartz straight pipe section 42 penetrating through the refractory blocks 24 and 25 forming the ceiling of the heating chamber 2 and an expanded pipe section expanded in the heating chamber 2. The small circular plate 8 made of quartz is supported and fixed inside the expanded tube portion 41 made of quartz by a small-diameter quartz tube 81 which stretches the inner surface of the expanded portion. The current plate 8 is arranged coaxially with and perpendicular to the axis.
These members may be made of heat resistant steel or the like instead of quartz, but are preferably made of quartz from the viewpoint of rapid uniform heating in the furnace.

In FIG. 1, an argon gas inlet 44 is provided in the furnace lid portion 11 with a straight pipe portion 42 of the argon gas injection pipe 4.
And is connected to an argon pipe 43. Further, the furnace lid 11 is provided with a terminal 71 for fixing the thermocouple protection tube 7 for temperature measurement in the heating chamber 2 and a terminal 52 of the conducting wire 51 of the nichrome electric heating element 5.

Also, in FIG.
An exhaust hole 61 connected to the vacuum pipe 6 and a viewing window 28 are provided.

FIG. 3 is a partial sectional view of the furnace bottom of the present apparatus. In this example, the substrate 31 of the baking table 30 is placed on a lifting table (table) 33 fixed to a lifting apparatus (not shown). ,
It is held so as to be able to be urged upward through a compression spring 35, and when the lift 33 is fixed at the upper limit position, the O-ring 3
11 is pressed against the outer bottom surface of the furnace shell bottom 101, and the substrate 31
And the furnace shell 10 is airtight. In this state, when the furnace pressure becomes higher than the atmospheric pressure, the furnace pressure presses the substrate 31 to the outside, that is, the lower side, compresses the compression spring 35, and the O-ring 311 separates from the outer surface of the bottom portion 101. A gas flow path is formed as shown by an arrow in the figure to reduce the internal pressure.

The relief valve mechanism using this compression spring
When the pressure is restored from the reduced pressure firing process due to the argon injection to the ceramic tooth, the inside of the furnace shell is prevented from becoming a pressure higher than the atmospheric pressure. It has a function of preventing the flow of argon gas from being changed painfully.

FIG. 4 shows a piping system and a control system of the present apparatus. The piping system is an argon gas introduction pipe 43
Is connected from an argon gas cylinder 47 to an argon on-off valve 45 via a flow meter 46 and the like.
4 is supplied with argon gas.

The vacuum pipe 6 is branched, and one pipe is connected to an atmosphere shielding valve 64 and the other pipe is connected to a vacuum pump 63 via a vacuum valve 62.

With respect to the control system, a temperature signal is input to a control unit 9 having a microcomputer for storing and executing a setting program through a lead wire 72 from a thermocouple, and a heating element is supplied from a mounting unit 91 in accordance with an instruction signal. 5 and the opening / closing operation of the argon on / off valve 45, the air shielding valve 64, and the vacuum valve 62.

Next, an embodiment of the production method of the present invention will be described.
explain. In the production of porcelain teeth, the sintering temperature is the highest in the opaque treatment. In this case, when ordinary porcelain is used, the maximum sintering temperature is about 950 ° C. The process of firing the porcelain teeth using this ordinary porcelain material and metallic titanium as a metallic denture is shown in the flowchart of FIG.

In the preparatory stage, the heating element 5 in the furnace is energized to maintain the atmospheric temperature in the heating chamber 2 at about 450 ° C. with the baking table 30 raised, and then the baking table 30 is lowered. As described above, a certain number of porcelain teeth, which are objects to be baked, obtained by applying porcelain to a titanium metal denture and drying the porcelain,
It is locked and fixed to the locking small bar 32 on the upper surface, then the baking table 30 is raised, and ceramic teeth are charged into the heating chamber 2 in the furnace to perform preliminary drying.

In this preliminary drying, the firing table 30 is gradually or stepwise raised to gradually increase the temperature of the porcelain teeth to prevent cracks and peeling of the applied porcelain in the dehydration and drying process.

When the baking table 30 is fixed at the upper limit and the opening at the bottom of the furnace is sealed, the atmosphere in the furnace is replaced with argon gas. This operation is performed by opening the vacuum valve 62 and evacuating by the vacuum pump 63. Then, the pressure inside the furnace was reduced to 10 mmHg or less, particularly to 0.1 mmHg or less, then the vacuum valve was closed, the argon gas opening / closing valve 45 was opened, and the argon gas introduction pipe 43 and the argon gas were introduced into the furnace. Argon gas is introduced through the injection pipe 4 to near atmospheric pressure. When this substitution operation of depressurization and argon introduction is performed twice, the argon concentration in the furnace atmosphere becomes 99% or more, and oxygen and nitrogen harmful to titanium of the denture body are diluted to 1% or less. In the replacement step, the temperature of the inside of the furnace and the temperature of the porcelain teeth are pre-heated while being kept substantially constant at around 450 ° C.

Next, the vacuum valve 62 is opened, the argon atmosphere in the furnace is reduced in pressure, and the electric power supplied to the heating element is increased to fire the porcelain teeth.
While maintaining the pressure of mmHg, the temperature was raised at a rate of about 60 ° C /
Heat the furnace rapidly to 950 ° C in min. During the firing process, the powdered and fixed porcelain softens and melts, and the bubbles generated during the melting escape and disappear in a reduced-pressure atmosphere, and after cooling, both the surface and the inside are sound-free and transparent. High ceramic quality.

As soon as the temperature in the furnace reaches the firing temperature of the porcelain 950 ° C., the argon on-off valve is opened, and argon gas is injected from the argon gas injection pipe 4.
The cooled argon gas is blown onto the surface of the porcelain teeth 3 on the firing table 30 from the expanded tube portion 41, so that the high-temperature porcelain teeth are rapidly cooled to at most about 450 ° C. If the temperature is kept at this temperature for a long time even after reaching the firing temperature, the molten surface porcelain will hang down or drip and deform the porcelain of the porcelain teeth. There is. This cooling time is about 2.5 min in this example. Simultaneously, the furnace pressure rises to the atmospheric pressure and is restored by the injection of the argon gas. Since the relief valve mechanism provided on the substrate 31 of the baking table 30 operates as described above with respect to the furnace pressure higher than the atmospheric pressure due to the introduction of argon gas, the inside of the furnace does not become high pressure. Further, the air shielding valve 64 may be opened.

If there is no rectifying plate 8 of the injection pipe 4, FIG.
As shown in (B), the argon gas ejected from the straight pipe portion 42 flows in the direction of the baking table 30 while keeping the jet diameter substantially equal to the inner diameter of the straight pipe portion 42, but does not spread inside the expanded tube portion 41. ,
Therefore, the argon gas flow reaching the entire area around the upper surface of the baking table 30 is weak. However, if the rectifying plate 8 is attached with an appropriate gap 80 provided, argon gas flows through the gap 80 between the peripheral edge of the rectifying plate 8 and the inner wall of the expanded portion 41 as shown in FIG. Under reduced pressure, it can be diffused as shown by the solid arrow and sprayed over the entire upper surface of the firing table 30. As the pressure approaches the atmospheric pressure, a phenomenon is observed in which the jet detours right below the central straightening plate 8 on the upper surface of the baking table 30 as indicated by a broken line arrow in the figure. By providing the current plate 8, a large number of ceramic teeth can be arranged on the upper surface of the firing table 30, and cooling can be performed almost simultaneously.

When the temperature of the ceramic teeth 3 has dropped to 450 ° C. or less, the firing table 30 is lowered to reduce the ceramic teeth 100 in air.
After allowing to cool to below ℃, the ceramic teeth are removed from the firing table 30.
At this stage, the furnace atmosphere temperature is maintained at about 450 ° C.

In the above firing method, since the firing step of the ceramic teeth and the argon gas cooling step are performed in a vacuum atmosphere or an argon-substituted atmosphere, the titanium of the denture body is oxidized even at a high temperature of 950 ° C. Never. Also,
Since the cooling process during the injection of the argon gas is also a low temperature of 450 ° C. or less in the cooling process after descending from the baking table, the surface is not oxidized even in the cooling process in the air. Although the above manufacturing method described the opaque treatment of the ceramic tooth, the body treatment, the stain treatment, and the glazing treatment,
It is performed similarly. In these cases, the material of the porcelain is changed, and firing is performed under reduced pressure at a firing temperature determined by the material.

Although the above embodiment is an example of a vertical furnace, a horizontal furnace may be used. In this case, if the heating chamber is made hollow so that the central axis is horizontal, the ceramic tooth holder may be, for example, a fire-resistant door openable door provided at a furnace shell side wall opening communicating with the heating chamber. The inner surface is used. The denture body or the porcelain tooth is locked by a fire-resistant rod protruding from the fire-resistant inner surface,
The inert gas injection pipe is arranged and fixed with the injection port of the pipe end opening facing the inner surface of the refractory. In addition, the furnace type, the structure of the ceramic tooth holder, the structure and arrangement of the inert gas injection pipe, and the like can be appropriately changed and used without departing from the scope of the present invention.

[0044]

The method and the apparatus for producing porcelain teeth according to the present invention are all carried out in an inert gas atmosphere or a reduced pressure atmosphere obtained by reducing the inert gas atmosphere in the steps of preheating, firing and cooling of the porcelain teeth. Therefore, even if the denture body constituting the porcelain tooth is an oxidizing metal, no oxide film is formed on the back surface and no oxide layer is formed at the boundary with the porcelain part. The material portion is not cracked and damaged, and the inside surface of the ceramic tooth can be attached to the tooth portion with extremely high accuracy.

In particular, in the cooling step after the firing step, the ceramic tooth whose surface is in a high-temperature molten state is cooled by blowing an inert gas or a nitrogen gas.
The porcelain tooth with a small heat capacity rapidly reduces the temperature and can be quickly taken out of the furnace in a non-oxidized state, thus preventing porcelain tooth deformation due to melting and dripping of the surface ceramic of the porcelain tooth, and It is possible to completely prevent the oxidation of the ceramic denture by the conventional cooling in the atmosphere. In addition, since the inside of the furnace can always be maintained at the preheating temperature, the ceramic teeth can be sequentially fired without cooling the inside of the furnace, and the productivity of the ceramic teeth is excellent.

The gas injection pipe is expanded toward the injection port,
When the expansion pipe is provided with a disk-shaped heat flow plate, the injected inert gas or nitrogen gas is spread radially, and it is possible to simultaneously cool a plurality or a large number of ceramic teeth on the firing table. Therefore, this device can easily bake a large number of ceramic teeth, and is excellent in mass productivity.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a ceramic tooth firing apparatus.

FIG. 2 is a partial perspective view (A) of a heating chamber of a ceramic tooth firing apparatus and conceptual views (B, C) showing jets from an inert gas injection pipe.

FIG. 3 is a sectional view of a bottom portion of the ceramic tooth firing apparatus.

FIG. 4 is a control system diagram of the ceramic tooth firing apparatus.

FIG. 5 is a flowchart of a ceramic tooth manufacturing process according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Firing furnace main body 2 Heating chamber 22 Quartz tube 23 Heat resistant body 3 Ceramic tooth 30 Firing table 31 Firing table substrate 4 Argon gas injection pipe 41 Expanding section 42 Straight pipe section 43 Argon gas introduction pipe 5 Nichrome heating element 6 Exhaust pipe 8 Heat flow plate 9 Control unit

Claims (6)

(57) [Claims] 1. A method for manufacturing a porcelain tooth in which a porcelain coated on a denture body surface made of an oxidizable metal is baked in a heating furnace, wherein the porcelain tooth or the porcelain locked on a porcelain tooth holder is coated. After inserting the attached denture body into the heating furnace, replacing the atmosphere in the furnace with a non-oxidizing atmosphere, and depressurizing the non-oxidizing atmosphere in the heating furnace and heating the inside of the heating furnace And then firing the ceramic teeth on the ceramic tooth holder in the heating furnace at a high temperature.
Blowing a non-oxidizing gas onto the ceramic tooth while holding
A cooling step of rapidly cooling the porcelain tooth to a temperature at which the denture body does not substantially oxidize in the atmosphere and returning the pressure to atmospheric pressure; and after cooling, the porcelain tooth holder and the porcelain tooth from the furnace. And a take-out step of taking out the product. 2. The method according to claim 1, wherein the non-oxidizing gas is an inert gas or a nitrogen gas. 3. An airtight heating furnace main body connected to a gas introduction pipe for introducing a non-oxidizing gas and a vacuum pipe, and a heating element surrounded by a refractory heat-resistant body inside the heating furnace main body. A heating chamber comprising: a heating chamber comprising: a ceramic tooth holder which holds the ceramic teeth to be heated and baked in the heating chamber; and a ceramic tooth holder which is inserted and fixed into the heating chamber;
And a gas injection pipe for jetting a non-oxidizing gas toward the ceramic tooth held by the ceramic tooth holder to rapidly cool the fired ceramic tooth. 4. A refractory base in which a bottom of the heating furnace body is provided with a furnace body opening communicating with the heating chamber, and the ceramic tooth holder is fixed on a substrate held on an elevating device. a firing table consisting of, the firing stage is from the furnace body opening
It is placed in the heating chamber and placed just below the end of the gas injection pipe
It is Rutotomoni apparatus of claim 3 in which the substrate was made to close the opening in contact with the edge of the furnace body opening. 5. A large number of small locking rods stand on the firing table.
The number of the porcelain teeth engageable been set arranged, the pipe end of the gas injection pipe has an expanded pipe portion which is Utate拡toward the firing table top, the inside of the expanded pipe portion, the The apparatus according to claim 3, wherein a flow straightening plate having a surface perpendicular to the central axis of the expanded portion is provided with a desired gas flow gap between the straightening plate and an inner wall of the expanded portion. 6. The apparatus according to claim 3, wherein the non-oxidizing gas is an inert gas or a nitrogen gas.