A melt-pouring system controlling the amount of a metal melt cast into a mold from a nozzle of a bottom-pouring-type ladle by opening and closing an upper opening of the nozzle in the ladle bottom by a stopper rod is widely used in casting, because it is advantageous in permitting less inclusions floating on the melt in the ladle to enter the mold.
FIGS. 10(a) and 10(b) schematically show a conventional bottom-pouring-type ladle. This bottom-pouring-type ladle 21 comprises a ladle body 2, a nozzle 3 provided in a bottom portion of the ladle body 2, a stopper rod 4 for closing the nozzle 3, an arm 5 supporting the stopper rod 4, and an elevating mechanism 6 for vertically moving the arm 5. The nozzle 3, which is usually formed by heat-resistant ceramics, has a reverse-conically tapered surface, or a spherically tapered surface having a convexly arcuate cross section. The stopper rod 4 is usually constituted by a sleeve 41 made of refractory materials such as graphite, and a metal-made core shaft 42 supporting the sleeve 41. The sleeve 41 usually has a reverse-conically tapered or semispherical lower end portion 41a. The arm 5 is constituted by a vertical arm portion 5a and a horizontal arm portion 5b, and the core shaft 42 is threadably attached to a tip end portion of the horizontal arm portion 5b with support members 7. In the depicted example, the nozzle 3 has an upper opening 10 having a spherically tapered surface 3a with an inward projecting fan-shaped cross section, and the stopper rod 4 has a semispherical lower end portion 41a. 
As shown in FIG. 10(a), when the stopper rod 4 is separate from the nozzle 3, a centerline O2 of the stopper rod 4 is substantially aligned with a centerline O1 of the nozzle 3. With the stopper rod 4 moving downward by the elevating mechanism 6 in this state, as shown in FIG. 10(b), the semispherical lower end portion 41a of the stopper rod 4 comes into close contact with the spherically tapered surface 3a of the nozzle 3, thereby closing the upper opening 10. In this state, a melt (not shown) is poured into the ladle body 2.
After the melt is poured into the ladle body 2 in the closed state shown in FIG. 10(b), the stopper rod 4 is lifted for a predetermined period of time as shown in FIG. 10(a) to discharge a predetermined amount of a melt through the nozzle 3, and then the stopper rod 4 is moved downward again. Because the centerline O2 of the stopper rod 4 is substantially aligned with the centerline O1 of the nozzle 3, the nozzle 3 must be closed. However, it is actually likely that the leakage of a melt through the nozzle 3 takes place in the state shown in FIG. 10(b). It has been found that the leakage of a melt through the nozzle 3 tends to increase as melt-pouring cycles are repeated.
When more than an acceptable amount of a melt is poured into the mold by leakage, or when a melt leaking before the start of pouring flows into the mold, defects called melt ball and cold shut may occur. Though the stopper rod 4 may be strongly pushed to the nozzle 3 with a large load, it would likely break the heat-resistant sleeve 41 of the stopper rod 4 or the nozzle 3.
As a result of intensive research to solve such a problem as the leakage of a melt, it has been found that (a) while a melt is discharged, not only inclusions in the melt but also a semi-solid melt are attached to the spherically tapered surface 3a of the nozzle 3, that (b) the inclusions and the semi-solid melt attached to the spherically tapered surface 3a of the nozzle 3 hinder the semispherical lower end portion 41a of the stopper rod 4 from coming into close contact with the spherically tapered surface 3a of the nozzle 3, and that (c) when a load necessary for downward movement while crushing or sliding the inclusions and the semi-solid melt attached to the spherically tapered surface 3a of the nozzle 3 is applied to the stopper rod 4, one or both of the semispherical lower end portion 41a of the stopper rod 4 and the nozzle 3 are likely damaged.
To cope with such a problem, JP 3-124363 A discloses, as shown in FIG. 11, a melt-pouring apparatus for supplying a predetermined amount of a melt from a decanting-type ladle to a basin 16, and then pouring this melt into a sprue 54 of a mold 41 with a melt-dropping nozzle 51 of the basin 16. This melt-pouring apparatus comprises a sand mold nozzle 53 in an upper portion of the mold 41, which is separate from the basin 16 and comes into close contact with the melt-dropping nozzle 51 of the basin 16; the sand mold nozzle 53 having the sprue 54; and the sprue 54 having a stopper-abutting seat 55 closely engageable with a stopper 25 entering the melt-dropping nozzle 51 of the basin 16. With this melt-pouring apparatus, without applying a large load to the stopper 25, the stopper 25 can come into highly close contact with the sand mold nozzle 53. However, the melt-pouring apparatus of JP 3-124363 A is an apparatus introducing a melt into the basin 16 from the decanting-type ladle, and then controlling the amount of a melt poured from the basin 16 to the mold 41, but not an apparatus controlling the amount of a melt poured from a bottom-pouring-type ladle. Accordingly, the nozzle 53 coming into contact with the stopper 25 is part of the sand mold, free from the problem of inclusions and a semi-solid melt attached.
Japanese Utility Model Publication No. 1-28944 discloses, as shown in FIG. 12, an apparatus for opening an outlet of a melt container, which comprises a main frame 112; two arms 104, 105 pivotally supported by the main frame 112; a frame 101 pivotally mounted to tip ends of the arms 104, 105; a driving means 108 fixed to the frame 101; an on-off rod 102 moved back and forth by the driving means 108; a plug 103 fixed to a tip end of the on-off rod 102; an arm-swinging means 106 pivotally supported by the main frame 112; links 109, 110 moving back and forth by the arm-swinging means 106 and pivotally connected to the main frame 112 and the arm 105; and a melt container outlet 111, into which the plug 103 of the on-off rod 102 is inserted; the plug 103 moving along a circular locus by two arms 104, 105 and the on-off rod 102, so that it comes into contact with an upper inner surface of the outlet 111, and then with the entire outlet 111. This outlet-opening apparatus is suitable for an aluminum melt, using a conical plug 103 to a cylindrical outlet 111. However, because the cylindrical outlet 111 does not have a tapered opening, the conical plug 103 is always in contact with an upper edge of the outlet 111, resulting in large wear. In addition, the contact of the cylindrical outlet 111 with the conical plug 103 does not provide sufficient closing, failing to prevent leakage when closed.