Dual-chambered molten metal holding furnace for low pressure casting

A dual-chambered molten metal holding furnace is for low pressure casting; producing cast products such as aluminum alloys using a low pressure casting method; and prevention of the gas release to the molten metal and the occurrence of air bubbles in the molten metal even when pressurized gas enters a material constituting the molten metal storage container. The part of a pressurizing chamber excluding a pressurizing pipe and a molten metal output pipe is opened to the atmosphere via an air passage gap positioned above a fixed molten metal surface level position L3. The air passage gap is positioned above the fixed molten metal surface level position L3. Even if pressurized gas is seeped into the material constituting a molten metal storage container via cracks or cracking subsequently occurred in the pressurizing pipe or minute gap originally present in the pressurizing pipe, the pressurized gas seeped from the air passage gap is released to the outside of the furnace.

PRIORITY CLAIM

This application is a national stage application, filed under 35 U.S.C. §371, of PCT Patent Application Serial No. PCT/JP2014/068987 filed on Jul. 17, 2014, entitled “Dual-Chambered Molten Metal Holding Furnace For Low Pressure Casting”, which is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a dual-chambered molten metal holding furnace for low pressure casting that is preferable for producing cast products such as aluminum alloys by means of the low pressure casting method.

Description of the Related Art

A dual-chambered molten metal holding furnace for low pressure casting including a molten metal storage container that partitions a molten metal holding chamber and a pressurizing chamber on the inside thereof and is formed of a monolithic refractory material; a cover plate made of a steel product that covers a bottom surface, a side surface, and a top surface of the molten metal storage container via a heat insulation layer and/or a fireproof layer around its circumference; a molten metal passage opening that is provided between the molten metal holding chamber and the pressurizing chamber; a lifting cutoff valve that opens and closes the molten metal passage opening; and tube heaters that are placed, respectively, in the interior of the molten metal holding chamber and the interior of the pressurizing chamber, wherein the pressurizing chamber includes a pressurizing part and a molten metal output part which are communicated with each other at the bottom thereof, a pressurizing pipe and a molten metal output pipe, each of which is a heat-resistant integral sintered product having impermeability and molded from fine ceramics or the like, are mounted inside the pressurizing part and the molten metal output part, respectively, is known (see Patent Document 1 with the applicant being the same as the present applicant). Although the monolithic refractory material constituting the molten metal storage container is permeable, an upper space of the molten metal surface level in the pressurizing chamber is in a completely sealed structure by the impermeable pressurizing pipe so as to take measures for seeping the molten metal into the molten metal storage container.

A heat-resistant integral sintered product molded from a material having permeability to some extent has been employed as each of the pressurizing pipe and the molten metal output pipe. In this case, since the pressurizing pipe and the molten metal output pipe exhibit a slight permeability, pressurized gas enters a material constituting the molten metal storage container from the pressurizing pipe. After the pressurized gas is held in the material for some time, the pressurized gas is released again in the molten metal, resulting in generation of air bubbles in the molten metal and defective products. Thus, in the technique disclosed in Patent Document 1, impermeability obtained by employing fine ceramics or the like which may be very expensive as a material is intended to prevent entry of pressurized gas into the material constituting the molten metal storage container from the pressurizing pipe and generation of air bubbles in the molten metal in association with reemission of the pressurized gas.

PRIOR ART DOCUMENTS

Patent Documents

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

If cracks or cracking occurs in the pressurizing pipe due to physical impact upon maintenance or the like, expansion and contraction with change in temperature, and other causes in spite of the fact that an upper space of the molten metal surface level in the pressurizing chamber is in a completely sealed structure by using impermeable members such as fine ceramics as the pressurizing pipe and the molten metal output pipe disclosed in Patent Document 1, the pressurizing pipe loses its impermeability, which brings entry of pressurized gas into the material constituting the molten metal storage container from the pressurizing pipe during pressurization. Consequently, the gas entered into the material constituting the molten metal storage container is held for some time and then is released into the molten metal, resulting in generation of air bubbles in the molten metal. Air bubbles generated in the molten metal may cause defective products such as voids in the castings.

The pressurizing pipe and the molten metal output pipe of which the main component is alumina or the like instead of fine ceramics having impermeability exhibit a slight permeability but not sufficient as a monolithic refractory material constituting the molten metal storage container therearound. Hence, pressurized gas may be seeped/held from the pressurizing pipe to the porous material part of the molten metal storage container from the beginning, which may cause defective products such as voids due to release of pressurized gas from the molten metal storage container into the molten metal.

The present invention has been made in view of the aforementioned circumstances, and an object of the present invention is to prevent release of the gas to the molten metal and the occurrence of air bubbles in the molten metal by releasing the pressurized gas from the molten metal storage container to the outside of the furnace even when the pressurized gas enters a material constituting the molten metal storage container from the pressurizing pipe.

Means for Solving the Problems

The dual-chambered molten metal holding furnace for low pressure casting of the present invention includes a molten metal storage container that partitions a molten metal holding chamber and a pressurizing chamber on the inside thereof and is formed of a monolithic refractory material; a cover plate made of a steel product that covers a bottom surface, a side surface, and a top surface of the molten metal storage container via a heat insulation layer and/or a fireproof layer around its circumference; a molten metal passage opening that is provided between the molten metal holding chamber and the pressurizing chamber; a lifting cutoff valve that opens and closes the molten metal passage opening; and tube heaters that are placed, respectively, in the interior of the molten metal holding chamber and the interior of the pressurizing chamber, wherein the pressurizing chamber includes a pressurizing part and a molten metal output part which are communicated with each other at the bottom thereof, a pressurizing pipe and a molten metal output pipe, each of which is a heat-resistant integral sintered product molded from a material having impermeability or permeability to some extent, are mounted inside the pressurizing part and the molten metal output part, respectively, the remaining part of the molten metal storage container is opened to the atmosphere via an air passage part positioned above the fixed molten metal surface level position, whereby, even when pressurized gas enters a material constituting the molten metal storage container from the inner walls other than the molten metal output pipe and the pressurizing pipe of the pressurizing chamber, the pressurized gas is released to the outside of the furnace by means of the air passage part so as to prevent release of the gas to the molten metal and the occurrence of air bubbles in the molten metal.

The top surface covering part of the molten metal output part is screwed to the side surface covering part of the molten metal storage container in the cover plate with bolts or the like at appropriate intervals, so that an air passage part can be configured as a gap between the side surface covering part and the top surface covering part. It is preferable that such an air passage part be disposed on the side part of the cover plate on the pressurizing chamber side. As alternative means for screwing the top surface covering part to the side surface covering part, a top surface covering part of the molten metal output part is intermittent-welded to the side surface covering part of the molten metal storage container in the cover plate, so that an air passage part can be configured as a gap between the cover plates in the non-welded part. Apart from this, an opening may be perforated into the cover plate above the fixed molten metal surface level position with provision of socket members or the like so as to perform venting.

Effects of the Invention

According to the present invention, the parts of the molten metal storage container other than the pressurizing pipe and the molten metal output pipe are in communication with the atmosphere via the permeable molten metal storage container, permeability of a heat insulation layer and/or a fireproof layer around its circumference, and the air passage part. Even if pressurized gas leaks from the pressurizing pipe into the monolithic refractory material when the pressurizing pipe which is an impermeable heat-resistant integral sintered product loses its impermeability due to occurrence of cracks or cracking or when the heat-resistant integral sintered product having permeability to some extent is used instead of the impermeable fine ceramics as the pressurizing pipe, the pressurized gas can be released to the outside of the furnace by means of the air passage part. Thus, release of the gas to the molten metal and the occurrence of air bubbles in the molten metal, which may be caused as a result of entry of the pressurized gas from the pressurizing pipe of the pressurizing chamber into the material constituting the molten metal storage container, may be eliminated, resulting in eliminating a cause of defective products. In addition, since the air passage part is disposed above the fixed molten metal surface level position, leakage of the molten metal to the outside in a long span can be avoided by the cover plate made of a steel product (iron skin) provided at the outermost circumference in spite of some permeability of the molten metal storage container and the heat insulation layer and/or the fireproof layer around its circumference.

BEST MODES FOR CARRYING OUT THE INVENTION

Hereinafter, a description will be given of an embodiment of the present invention with reference to the attached drawings. InFIGS. 1 and 2, the reference numeral10denotes the entirety of the dual-chambered molten metal holding furnace for low pressure casting (hereinafter simply referred to as “molten metal holding furnace”) according to the present invention. The molten metal holding furnace10includes a molten metal storage container12, and the molten metal storage container12is molded by a monolithic refractory material. In this embodiment, the monolithic refractory material which serves as a material for the molten metal storage container12is mainly composed of, for example, powder alumina. The powder alumina is kneaded with water, is molded into a predetermined shape (casting), and then is cured and dried.

A fireproof layer14and a heat insulation layer16reside in this order on the outside of the molten metal storage container12, the bottom surface and side surfaces and a part of the top surface, all of which are outside the heat insulation layer16, are firmly covered by an iron skin18(a cover plate part for covering side surfaces and a top surface of the molten metal storage container in the cover plate made of a steel product in the present invention). The fireproof layer14is composed of a material such as alumina or other refractories, which is kneaded with water in appropriate ratios and then can be molded by molding and drying. The heat insulation layer16can be configured by attaching a fire-resistant fabric.

The internal space of the molten metal storage container12is partitioned into a molten metal holding chamber20and a pressurizing chamber22. A holding chamber lid24is mounted on the upper opening of the molten metal holding chamber20and a part of the holding chamber lid24is a replenishment opening lid26that is capable of being opened and closed to cover the molten metal replenishment opening. A level sensor28for detecting the upper-limit molten metal surface level L1of the molten metal in the molten metal holding chamber20is supported in a suspended state by the holding chamber lid24. The molten metal holding chamber20also includes two tube heaters30and a temperature sensor32at the side wall part thereof. With this arrangement, the molten metal holding chamber20can hold the molten metal accumulated therein within a certain temperature range. Note that the lower-limit molten metal surface level in the molten metal holding chamber20is shown by a chain-dotted line L2.

A lifting cutoff valve34extends vertically in the molten metal holding chamber20, and the lower end of the lifting cutoff valve34is positioned to face a molten metal passage opening36disposed between the molten metal holding chamber20and the pressurizing chamber22, and the molten metal passage opening36can be opened and closed by the lifting cutoff valve34. In other words, a valve seat38is securely-attached to the molten metal passage opening36so that the inflow of molten metal from the molten metal holding chamber20to the pressurizing chamber22is prevented when the lifting cutoff valve34is seated to the valve seat38and the inflow of molten metal from the molten metal holding chamber20to the pressurizing chamber22is permitted when the lifting cutoff valve34lifts from the valve seat38. The upper end of the lifting cutoff valve34protrudes outwardly through the holding chamber lid24, and the lifting cutoff valve34is connected to a lifting drive mechanism40in a pneumatic system or the like for controlling the opening and closing operations of the lifting cutoff valve34.

The pressurizing chamber22includes a pressurizing part44and a molten metal output part46which are communicated with each other at the bottom thereof via a lower flow passage42leading to the molten metal passage opening36. One end43-1of a tube heater43for maintaining a molten metal temperature in the pressurizing chamber22is secured to the furnace wall side, and the other end of the tube heater43cantileveredly extend into the lower flow passage42. Although the number of the molten metal output part46is one inFIG. 2, the molten metal output part46may also be provided in plural so as to supply molten metal from the common pressurizing part44.

The pressurizing part44and the molten metal output part46include tubular members48and50(hereinafter respectively referred to as “pressurizing pipe” and “molten metal output pipe”), respectively, for covering the inner surface of the molten metal storage container12. In this embodiment, the pressurizing pipe48and the molten metal output pipe50are formed by kneading powder fine ceramics or granulated fine ceramics (e.g., silicon nitride) with water and by integral sintering (sintering) after molding. Thus, the pressurizing pipe48and the molten metal output pipe50are impermeable in this embodiment. Cylindrical recesses44A and46A are cut and formed in the inner peripheral surface of the molten metal storage container12in the pressurizing part44and the molten metal output part46, and the pressurizing pipe48and the molten metal output pipe50are intimately fit via a sealing member so as to be flush with the cylindrical recesses44A and46A, respectively. The present invention does not exclude the possibility that the pressurizing pipe48and the molten metal output pipe50exhibit permeability to some extent (a very slight permeability to air remains but not as much as free permeability to air of the molten metal storage container12) in the embodiment to be described below.

The upper end flange part48-1of the pressurizing pipe48comes into engagement with the ceiling part18-1of the iron skin18around the entire periphery thereof and the opening of the pressurizing pipe48is sealed with a sealing lid52around the entire periphery thereof. In other words, a flange part52-1is formed on the outer periphery of the sealing lid52, a bolt53(a bolt with hex hole or the like) is inserted into the flange part52-1from above, and the leading end of the bolt53is screwed into the ceiling part18-1of the iron skin18. As shown inFIG. 2, the bolts53are provided along the entire periphery of the sealing lid52at appropriate intervals. A seal is provided for sealing at the interface between the ceiling part18-1of the iron skin18and the flange part48-1and at the interface between the flange part48-1and the sealing lid52. The upper end flange part48-1of the pressurizing pipe48is clamped between the flange part52-1of the sealing lid52and the ceiling part18-1of the iron skin18by the fastening (screwing) bolts53via a seal. Thus, the pressurizing part44is in a completely sealed structure at a part above the pressurizing pipe48in conjunction with the impermeability of the pressurizing pipe48consisting of fine ceramics. A pressurized gas passage54(to be connected to a pressurized gas source which is not shown) and a pair of level sensors56are provided on the sealing lid52, and a sensing end of each level sensor56vertically and cantileveredly extends into the internal cavity of the pressurizing part44. The fixed molten metal surface level L3in the pressurizing part44is detected by the level sensors56. The fixed molten metal surface level L3is set to the same height as the lower-limit molten metal surface level L2of the molten metal holding chamber20.

A ceiling plate58(the ceiling plate58and the ceiling part18-1of the iron skin18serve as the top surface covering part of the molten metal storage container in the cover plate according to the present invention) made of steel product is provided on the upper wall surface of the furnace at the molten metal output part46. A boss58-1is formed at the center of the ceiling plate58, the molten metal output pipe50is inserted through the boss58-1, the molten metal output pipe50is adapted to protrude somewhat from the boss58-1, and a die base (shown by a phantom line60) is connected to the ceiling plate58via an annular sealing member59. A liquid level L5in the molten metal output part46indicates the molten metal liquid level upon completion of preparation for output of molten metal to a die, and the liquid level L6indicates the molten metal liquid level upon completion of output of molten metal to a die. The molten metal output pipe50is positioned so as to extend downwardly below the liquid level L6. A mold (not shown) is secured on the lower die base60. The mold has a cavity inside thereof corresponding to casting and also has a molten metal communication passage for communicating the cavity to the molten metal output part46. During the filling of molten metal to the mold at the pressurizing part44, molten metal is pushed out by applying pressure to the molten metal surface by the pressurized gas introduced from the pressurized gas passage54so that the molten metal surface is lowered from the fixed molten metal surface level L5to the molten metal surface level L6but the lower end of the molten metal output pipe50is positioned below the molten metal surface level L6.

The ceiling plate58not only functions as a cover plate that covers the molten metal storage container12, the fireproof layer14, and the heat insulation layer16on the top surface of the molten metal output part46but also functions for die connection. Thus, the ceiling plate58is composed of the same steel material as the iron skin18but has a significant thickness in order to ensure a required strength. In other words, the ceiling plate58extends to the side wall part18-2of the iron skin18on the one hand, but extends to the side wall part18-3that extends vertically downward from the ceiling part18-1of the iron skin for covering the top surface of the pressurizing part44on the other hand.

A brief description will be given of the supply of molten metal by the molten metal holding furnace1. Firstly, the replenishment opening lid26is open by raising a cutoff valve34with the molten metal passage opening36being open so as to supply molten metal to the molten metal holding chamber20. The molten metal supplied to the molten metal holding chamber20enters and is stored in the pressurizing chamber22via the molten metal passage opening36. When the level sensors56detect that the molten metal surface in the pressurizing part44has finally reached the fixed molten metal surface level L3, the molten metal passage opening36is closed by lowering the cutoff valve34. At this time, the molten metal surface in the molten metal output part46has also reached the fixed molten metal surface level L5which is the same height as the fixed molten metal surface level L3. Furthermore, when the level sensor28detects that the molten metal surface has reached the upper-limit molten metal surface level L1by continuing to supply molten metal to the molten metal holding chamber20, the supply of molten metal is stopped and then the replenishment opening lid26is closed. With this arrangement, the casting process becomes ready for execution. Next, in the casting process, pressurized gas (e.g., dry air, N2 gas, Ar gas, or the like) is supplied from the pressurized gas passage54into the pressurizing part44, and then, a pressure of about 0.2 to 0.5 atm is applied to the molten metal surface so as to push upward the molten metal in the molten metal output part46. Consequently, the molten metal in the molten metal output part46is filled into the cavity of the mold. At this time, the molten metal surface in the pressurizing part44lowers from the fixed molten metal surface level L3to the molten metal surface level L7. After the elapse of a predetermined time from the completion of filling the molten metal into the mold, the pressure on the pressurizing part44is released to atmospheric pressure via the pressurized gas passage54. Although this leads to a return of the molten metal to the molten metal output part46, the molten metal in the molten metal storage container12is decreased by the amount required for one casting operation, and thus, the molten metal surfaces in the molten metal output part46and the pressurizing part44respectively become the molten metal surface levels L6and L4which are lower than the fixed molten metal surface levels L5and L3, respectively. Then, when the molten metal passage opening36is open by raising the cutoff valve34, the molten metal in the molten metal holding chamber20enters the pressurizing chamber22due to the difference in height between the molten metal surface level in the molten metal holding chamber20and that in the pressurizing chamber22. When the level sensors56detect that the molten metal surface level in the pressurizing part44has been raised and reached the fixed molten metal surface level L3, the molten metal passage opening36is closed by lowering the cutoff valve34. At this time, the molten metal surface in the molten metal output part46has also reached the fixed molten metal surface level L5which is the same height as the fixed molten metal surface level L3in the pressurizing part44. With this arrangement, the next casting process becomes ready for execution. By repetition of the casting process as described above, the molten metal in the molten metal holding chamber20is decreased successively and stepwisely. When the molten metal surface in the pressurizing part44does not rise to the fixed molten metal surface level L3by opening the molten metal passage opening36, the level sensors56cannot detect the fixed molten metal surface level L3. Thus, it can be determined that a time period for replenishment of the molten metal is reached, so that the molten metal is automatically or manually replenished in the molten metal holding chamber20by opening a molten metal replenishment lid26.

In the above embodiment, the pressurizing part44is in a completely sealed structure at a part above the pressurizing pipe48but the remaining part of the molten metal storage container12is not sealed. In other words, the molten metal storage container12is completely covered at its bottom wall part and its side surface part by the iron skin18via the fireproof layer14and the heat insulation layer16. However, the ceiling part18-1of the iron skin18and the side wall parts18-2and18-3thereof are not in a completely sealed structure. In other words, as shown inFIGS. 1 and 2, the ceiling part18-1of the iron skin18is simply screwed to the upper ends18-2′ and18-3′ of the side wall parts18-2and18-3, respectively, with bolts (bolts with hex hole or the like) at appropriate intervals and the ceiling part18-1of the ceiling plate58for covering the top surface of the molten metal storage container12around the molten metal output part46is also not completely sealed with respect to the side wall part18-2but is simply screwed to the side wall part18-2with bolts (bolts with hex hole or the like)64at appropriate intervals (seeFIG. 2). Thus, narrow gaps66and67(seeFIGS. 1 and 2) remain between opposite surfaces of the ceiling part18-1of the iron skin18and the ceiling plate58and the upper end18-2′ of the side wall part18-2of the iron skin18and between opposite surfaces of the ceiling part18-1of the iron skin18and the ceiling plate58and the upper end18-3′ of the side wall part18-3of the iron skin18, respectively. The gap66between the upper end18-2′ of the side wall part18-2of the iron skin18and the ceiling plate58is clearly shown inFIG. 3. Each of these gaps66and67serves as an air passage part that vents the permeable furnace materials, i.e., the molten metal storage container12, the fireproof layer14, and the heat insulation layer16to the outside air. In the present invention, the gaps66and67constituting the air passage part are provided uniformly distributed over substantially the entire side surface part (the iron skin18) of the cover plate on the pressurizing chamber22side (seeFIGS. 1 and 2), which is advantageous for the efficient discharge of gas seeped from the pressurizing pipe48to the molten metal storage container12. Such a ventilation structure prevents defective products such as voids due to air bubbles in the molten metal in association with reemission of the pressurized gas when the pressurized gas in the pressurizing pipe48is seeped and held to the p membrane material constituting the molten metal storage container12. In other words, in the present embodiment, the pressurizing pipe48is composed of an impermeable member such as a ceramics material, seepage of pressurized gas in the pressurizing pipe48toward the membrane material side does not occur under normal circumstances. However, if cracks or cracking occurs in the pressurizing pipe48which is the heat-resistant integral sintered product formed of impermeable fine ceramics due to physical impact upon maintenance or the like, and expansion and contraction with change in temperature and the pressurizing pipe48loses its impermeability, pressurized gas enters into the molten metal storage container12through the pressurizing pipe48of the pressurizing chamber22, the gas entered into the molten metal storage container12is held for some time and then is released into the molten metal, resulting in generation of air bubbles in the molten metal. In the present embodiment, seepage of gas from the pressurizing pipe48to the molten metal storage container12due to a loss of impermeability caused by cracks, cracking or the like occurred in the pressurizing pipe48can be avoided by releasing the gas seeped from the air passage gaps66and67formed by the fact that the ceiling part of the iron skin18is screwed to the upper end of the side wall part of the iron skin18with bolts at appropriate intervals to the outside of the furnace. Consequently, leakage of the gas to the molten metal and the occurrence of air bubbles in the molten metal, which may be caused as a result of entry of the pressurized gas from the inner walls of the pressurizing chamber into the molten metal storage container12, are eliminated. In the present embodiment, the pressurized gas entered into the molten metal storage container12is released from the air passage gaps66and67which are gaps formed by screwing, and thus, the pressurized gas entered into the molten metal storage container12is not released into the molten metal, resulting in no occurrence of air bubbles, so that a cause of defective products can be eliminated.

Furthermore, since the gap66with respect to the iron skin side wall surface18-2of the ceiling plate58is positioned above the fixed molten metal surface level L5, seepage of the molten metal to the outside via the permeable molten metal holding chamber20, the fireproof layer14, and the heat insulation layer16can be avoided in a long-term span. Although the flow of the molten metal, of course, crosses the fixed molten metal surface level L5upon output of the molten metal in the molten metal output part46, the speed of seepage of the molten metal caused by the permeability of furnace materials is extremely slow, the presence of the gap66does not cause seepage of the molten metal in a short-term span such as upon output of the molten metal in the molten metal output part46.

While, in the above embodiment, the parts of the molten metal storage container12other than the pressurizing part44are vented to the outside air by remaining narrow gaps, between opposite surfaces of the ceiling part18-1of the iron skin18and the ceiling plate58and the side wall part18-2of the iron skin18, formed by screwing with the bolts62and64, intermittent-welding may be used instead of screwing with bolts in the second embodiment.FIG. 4is a general view illustrating a dual-chambered molten metal holding furnace for low pressure casting according to the second embodiment, the ceiling part18-1of the iron skin18and the ceiling plate58are secured to the side wall part18-2of the iron skin18with welding parts68and70, respectively, instead of bolts62and64shown inFIG. 1. This welding is so-called “intermittent-welding” at predetermined intervals, the intermittent-welded parts70of the ceiling plate58with respect to the upper end of the side wall part18-2of the iron skin18are shown inFIG. 5, narrow gaps72between the ceiling plate58and the iron skin side wall surface18-2remain between the intermittent-welded parts70, which serve as air passage parts. Although not illustrated, the same gaps constituting the air passage parts at non-welded parts between the intermittent-welded parts68also remain between the ceiling part18-1and the side wall part18-3of the iron skin18. In other words, the gaps72constituting the air passage parts are disposed over a wide range of the side surface part of the cover plate (the iron skin18and the ceiling plate58) on the pressurizing chamber22side, and thus, the gas seeped from the pressurizing pipe48to the molten metal storage container12can also be efficiently discharged to the outside of the furnace in this embodiment. As in the first embodiment, when the pressurized gas leaks from the pressurizing pipe48into the molten metal storage container12due to the occurrence of cracks or cracking, the gas is released from the air passage part to the outside of the furnace, so that release of the gas to the molten metal and the occurrence of air bubbles in the molten metal may be prevented.

FIG. 6is a partial view illustrating an essential part of a dual-chambered molten metal holding furnace for low pressure casting according to a third embodiment. Although the ceiling part18-1of the iron skin18and the ceiling plate58are welded with respect to the side wall part18-2of the iron skin18as in the second embodiment, this welding is applied to the entire periphery thereof, and thus, a socket74(hole-formed member) is provided at the side wall18-2of the iron skin18above the fixed molten metal surface level L5(L3) for ventilation. The presence of the socket74allows a part other than the pressurizing part44in the molten metal holding chamber20to be opened to the atmosphere due to permeability of furnace materials above the fixed molten metal surface level L5(L3). Thus, as in the first and second embodiments, when the pressurized gas leaks from the pressurizing pipe48into the molten metal storage container12, the gas is released from the air passage part to the outside of the furnace by the effect of the socket74, so that release of the pressurized gas to the molten metal and the occurrence of air bubbles in the molten metal may be prevented. As in the gaps66and72in the first and second embodiments, the socket74is also disposed over a wide range of the side surface part of the cover plate (the iron skin18and the ceiling plate58) on the pressurizing chamber22side, and thus, the pressurized gas can be efficiently discharged to the outside of the furnace.

While, in the first to third embodiments of the present invention described above, the pressurizing pipe48and the molten metal output pipe50are composed of fine ceramics as a material, the present invention also encompasses the case, as a fourth embodiment, where the pressurizing pipe48and the molten metal output pipe50are formed by kneading a fire-resistant powder consisting of alumina, silica, carbon, and the like with water and by integral sintering (sintering) after molding so as to give some permeability to the pressurizing pipe48and the molten metal output pipe50. In other words, since a completely sealed structure is not obtained in this case, seepage of pressurized gas may occur from the pressurizing pipe48to the porous membrane material part from the beginning. The provision of air passage parts (the gaps66,67, and72and the socket74) having the same structure as in the first to third embodiments where the pressurizing pipe48and the molten metal output pipe50exhibit impermeability allows gas to be released to the outside of the furnace via the air passage parts even if seepage of gas occurs from the pressurizing pipe48to the molten metal storage container12, so that the occurrence of defective products may be avoided.

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