COUNTERGRAVITY CASTING APPARATUS

Abstract: Apparatus for the shaping of metal in a bottom-gated mould 6 by a vacuum-induced countergravity casting method, which mould includes an upstanding levee 34 on a periphery of a drag portion 10 which circumscribes a joint between the drag portion 10 and a cope portion 8 for isolating said joint when the mould 6 is immersed in an underlying melt 4.

Description: This invention relates to apparatus for shaping a metal article according to the preamble of claim 1. Background of the Invention The vacuum-induced, countergravity casting process is useful in the making of thin-walled near-net-shape castings and involves: sealing a bottom-gated mould, having a gas-permeable upper portion, to the mouth of a vacuum chamber such that the chamber confronts the upper portion; immersing the underside of the mould in an underlying melt; and evacuating the chamber to draw melt up into the mould through one or more of the gates in the underside thereof. Such a process is shown in U.S. patent 4,340,108 (Chandley et al) wherein the mould comprises a resin-bonded-sand shell having an upper cope portion sealingly bonded (e.g., glued) to a lower drag portion and the vacuum chamber sealed on top of the cope so that the parting line between the cope and drag lies outside the vacuum chamber. U.S. patent 4,632,171 (Almond) is an improvement on U.S. patent 4,340,108 and seals the mould to the vacuum chamber on top of the drag so that the parting line between the cope and drag falls within the vacuum chamber. In such processes, it is desirable to immerse the drag as far as possible into the melt to prevent invasion of the mould cavity by air being sucked through any portions of the porous drag which might be above the melt surface. Hence, in both the U.S. patents quoted above, the parting line between the cope and the drag is brought into close proximity to the surface of the underlying melt during casting. Moreover, in U.S.4,632,171, the seal between the vacuum chamber and the mould is also brought into close proximity to the surface of the underlying melt during immersion. It is undesirable to have melt contact either the parting line between the cope and the drag or the joint between the vacuum chamber and the mould. In this regard, melt at the parting line: (1) can cause gasification of any glue therein which gases are drawn into the mould cavity and can be trapped in the casting; and (2) can be sucked into the mould cavity at the parting line thereby ruining the casting. Moreover, hot melt at the chamber-mould joint can be sucked directly into the vacuum chamber causing ignition of the gases therein and result in destruction of the mould, casting and possibly the chamber itself. It is not always possible to control precisely the depth that the mould is immersed into the underlying melt. Hence there is a risk that the cope-to-drag parting line and/or the chamber-to-mould joint may accidentally be contacted by the hot melt unless some technique can be devised for isolating them from the melt under all circumstances. It is an object of the present invention to provide improved apparatus for the vacuum-induced, countergravity casting of metal into porous, bottom-gated moulds wherein the cope-to-drag parting line and/or the chamber-to-mould joint is isolated from the melt over a wide range of immersion depths. This and other objects and advantages of the present invention will become more readily apparent from the description thereof which follows. Brief Description of the Invention Essentially the invention comprehends countergravity casting apparatus of the aforesaid type wherein the drag includes a peripheral levee circumscribing the chamber-to-mould joint and/or the cope-to-drag parting line to prevent melt from the underlying melt from contacting the joint or parting line over a wide range of mould immersion depths including depths where the chamber-to-mould joint and/or the cope-to-drag parting line are beneath the surface of the melt during casting. The levee formed on the drag permits melt to rise high up along the sides of the mould without invading the parting line or joint and making deleterious incursions into either the mould cavity or vacuum chamber. In the case of resin-bonded-sand moulds, the levee will preferably be formed on the drag by moulding against a drag-shaping pattern in a similar manner to the drag-forming method disclosed in U.S. patent 4,632,171 which is intended to be incorporated herein by reference as it relates to such method. Brief Description of the Drawings The invention will better be understood when considered in the light of the following detailed description of certain embodiments of the invention which are described hereafter in conjunction with the accompanying drawings, in which: Figure 1 is a partial side-sectional view (i.e., in the direction 1-1 of Figure 2) through a vacuum-induced, countergravity metal casting apparatus in accordance with the present invention; Figure 2 is a plan view of the apparatus of Figure 1; (i.e., in the direction 2-2 of Figure 1) and Figure 3 is a partial side-sectional view, like that of Figure 1, but of another embodiment of the present invention. Figures 1 and 2 show a vessel 2 of metal melt 4 which is to be drawn up into a mould 6. The mould 6 include a gas-permeable, upper (i.e., cope) portion 8 joined (e.g., glued) to a lower (i.e., drag) portion 10 along a parting line 12 which define therebetween a moulding cavity 16. The lower drag portion 10 includes a plurality of in-gates 14 on the underside thereof for supplying melt to the mould cavity 16 when the cavity is evacuated. The drag portion 10 of the mould 6 is sealed to a lip 18 at the mouth of a vacuum chamber 20 so that the gas-permeable upper portion 8 is encompassed by the chamber 20. The vacuum chamber 20 is connected to a vacuum source (not shown) via conduit 22. The upper portion 8 of the mould 6 comprises a gas-permeable material (e.g., resin-bonded-sand) which permits gases to be withdrawn from the mould cavity 16 when a vacuum is established in the chamber 20. The lower portion 10 of the mould 6 may conveniently comprise the same material as the upper portion 8, or other materials, permeable or impermeable, which are compatible with the upper portion material. The mould 6 may be secured to the chamber 20 by means of inverted threaded shafts 24 which are screwed into upstanding mounting lugs 26 in the manner described in U.S. patent No. 4,658,880 (Voss). Other methods of securing the mould to the chamber are acceptable as well. The drag portion 10 of the mould 6 includes a peripheral flange 28 which extends outside a peripheral edge 30 of the cope 8. The flange 28 includes: (1) a flat upper surface 32 for sealingly engaging the lip 18 of the chamber 20 (e.g., via a gasket of Fiberfax [not shown] insulating material); and (2) an upstanding levee 34 circumscribing the upper surface portion 32 (and hence the joint between the lip 18 and the surface 32). As shown in Figure 1, the levee 34 permits immersion of the mould so deep in the melt 4 that the joint between the chamber 20 and the surface 32 lies beneath the surface 36 of the melt yet is isolated from the melt by the levee 34. Figure 3 depicts another embodiment of the present invention wherein a lip 38 on the mouth of a chamber 40 sealingly engages a flat surface 42 on top of a cope 44. The cope 44 engages a drag 46 along a parting line 48. The drag 46 has a flange 50 extending outside the cope 44, which flange includes an upstanding levee 52 circumscribing the parting line 48 and isolating it from contact with a melt 54 as described above. While the invention has been disclosed in terms of specific embodiments thereof it is not intended to be limited thereto but rather only to the extent set forth hereafter in the claims which follow.

Claims: 1. Apparatus for shaping a metal article in a bottom-gated mould (6) by a vacuum-induced, countergravity casting method wherein the mould (6) is immersed in an underlying melt (4; 54) of said metal and evacuated to draw said melt (4; 54) up into said mould (6), said apparatus comprising: a gas-permeable cope portion (8; 44) of said mould defining a mould cavity (16) for shaping said article, said cope portion (8; 44) having a peripheral edge (30) defining a first sealing face on its underside between said edge (30) and said cavity (16); a drag portion (10; 46) of said mould (6) further defining said cavity (16) and having a second sealing face on its topside surrounding said cavity (16); means for sealingly engaging said first and second sealing faces one to the other at a parting line (12; 48) therebetween; a vacuum chamber (20; 40) on top of said drag portion (10; 46) and enclosing said cope portion (8; 44), said chamber (20; 40) having a lip (18; 38) on its underside sealingly engaged with said mould (6); and means (22) for evacuating said chamber (20; 40) sufficiently to draw said melt (4; 54) up into said mould (6) when it is immersed in said melt (4; 54), characterised in that a flange (28; 50) on said drag portion (10; 46) extends outside said second sealing face beyond said peripheral edge (30), said flange (28; 50) has a substantially flat upper surface portion (32) and an upstanding levee portion (34; 52) circumscribing said upper surface portion (32) to substantially isolate said upper surface portion (32) from said melt (4; 54) when said mould (6) is immersed in said melt (4; 54); and said vacuum chamber lip (18; 38) sealingly engages with said upper surface portion (32). 2. Apparatus according to claim 1, for shaping a metal article in a bottom-gated, expendable shell mould by the vacuum-induced, countergravity casting method, in which the cope portion (8) of said mould (6) comprises a gas-permeable, resin-bonded-sand shell formed by moulding against a cope-shaping pattern, the drag portion (10) of said mould (6) comprises a resin-bonded-sand shell formed by moulding against a drag-shaping pattern, and said substantially flat upper surface portion (32) and said upstanding levee portion (34) are formed against said drag-shaping pattern at the time of moulding said drag portion (10). 3. Apparatus according to claim 1 or 2, in which the upstanding levee portion (52) on the flange (50) extends upwardly above the thickness of the peripheral edge of said cope portion (44), and said vacuum chamber lip (38) sealingly engages with a flat top surface (42) of said peripheral edge of the cope portion (44) instead of with said upper surface portion of the drag portion (46).

Inventors: MERCER, JAMES B., VOSS, KARL D.
Applicants: GENERAL MOTORS CORPORATION
Application year: 1988
Main classification: B22D18
IPC codes: B22C9, B22D18