COUNTERGRAVITY CASTING APPARATUS

Abstract: Apparatus for the vacuum countergravity casting of metal including an elastomeric sealing gasket (28) substantially thermally insulated, conduction-wise, from the underlying heat of the molten metal (4) by the mould-forming material (l0) and shielded from the radiant heat of the molten metal (4) by a surrounding skirt (38) depending from a vacuum box (22).

Description: This invention relates to apparatus for the vacuum countergravity casting of metal in gas-permeable, shell moulds and, more particularly, to means for sealing a mould to a vacuum chamber. Background of the Invention The vacuum countergravity, shell mould casting process is particularly useful in the making of thin-walled castings and involves: sealing a bottom-gated mould, having a gas-permeable upper portion, to the mouth of a vacuum chamber so 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 US-A-4,340,l08, wherein the mould comprises a resin-bonded-sand shell having an upper cope portion and a lower drag portion sealingly bonded together. US-A-4,340,l08 seals the mould to the vacuum chamber on top of the cope, so that the parting line between the mould halves lies outside the vacuum chamber. Copending European patent application Serial No.86307265.8 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, and particularly that shown in European Serial No. 86307265.8 the gasket material used to seal the vacuum chamber to the mould is necessarily brought into close proximity to the surface of the underlying melt during casting. Hence the gasket material is exposed to the tremendous heat that radiates from the melt pot. As a result, only high temperature-resistant gasket material, such as Fiberfrax, (from the Carborundum Co.) has been used, heretofore. In this regard, Fiberfrax strips are typically glued to the surface of the mould and the mouth of the vacuum chamber pressed firmly against the material to compress it and form the desired mould-chamber seal. When so applied and used, extra time is required to manually affix the gaskets to each mould and the gasket material is ultimately destroyed with the mould following casting. It would be desirable if an elastomeric gasket material could be affixed to the mouth of the vacuum chamber for repeated use with many moulds. This would eliminate the time required to manually prepare each mould-chamber seal as well as the unnecessary consumption of gasket material. Unfortunately, elastomeric gasket materials cannot survive the kind of direct exposure to the radiant heat from the molten metal that the Fiberfrax seals have had to endure. It is an object of the present invention to provide improved apparatus for the vacuum countergravity casting of shell moulds wherein the vacuum chamber is sealed to the mould at a site which is substantially thermally insulated (i.e., conduction-wise) and shielded (i.e., radiation-wise) from the surface of the metal melt during casting so as to permit the repeated use of a thermally-degradable elastomeric gasket at the site. This and other objects and advantages of the present invention will become more readily apparent from the detailed description thereof which follows. Brief Description of the Invention The invention comprehends an improved vacuum countergravity casting apparatus including: a mould having a porous, gas-permeable upper shell and a bottom-gated lower portion secured to the upper shell; a sealing surface on top of the mould which is substantially insulated, conduction-wise, from the heat of the metal in the underlying melt pot; a vacuum box comprising a peripheral wall defining a vacuum chamber having a lip on the underside thereof defining a mouth of the chamber; an elastomeric gasket compressed between the underside of the lip and the sealing surface of the mould; and a skirt depending from the wall beneath the lip so as to surround the gasket and the sealing surface sufficiently to shield the gasket from heat radiating from the pot. The sealing surface on the mould may be formed on top of the upper shell. Preferably, however, the lower portion of the mould will include a continuous upstanding ridge lying outboard of the upper shell and adjacent the periphery of the mould, which ridge has the sealing surface of the mould formed on the top thereof. The thickness of the mould-forming material (e.g., resin-bonded-sand) between the gasket and the melt serves to insulate the surface, conduction-wise, from the heat of the pot. Detailed Description of Preferred Embodiment The invention may better be understood when considered in the light of the following detailed description of one specific embodiment thereof which is given hereafter in conjunction with the accompanying drawings, in which: Figure l is a side, sectioned view (i.e., in direction l-l of Figure 2) through a vacuum countergravity metal casting apparatus in accordance with the present invention; Figure 2 is a view in the direction 2-2 of Figure l; and Figure 3 is a view in the direction 3-3 of Figure 2. Figure l shows a pot 2 of metal melt 4 which is to be drawn up into a mould 6. The mould 6 includes gas-permeable, upper portions 8 and 8′ joined (e.g., glued) to a lower portion l0 along parting lines l2 and l2′ and define therebetween separate moulding cavities l6 and l6′ The lower portion l0 includes a plurality of ingates l4 on the underside thereof for supplying melt to the mould cavities l6 and l6′ when the cavities are evacuated. The lower portion l0 of the mould 6 is sealed to a mouth l8 of a vacuum chamber 20, which is defined by vacuum box 22, so that the gas-permeable upper portions 8 and 8′ are encompassed by the chamber 20. The vacuum chamber 20 is communicated to a vacuum source (not shown) via conduit 23. The upper portions 8 and 8′ of the mould 6 comprise a gas-permeable material (e.g., resin-bonded-sand) which permits gases to be withdrawn from the casting cavities l6 and l6′ when a vacuum is created in the chamber 20. The lower portion l0 of the mould 6 may conveniently comprise either the same material as the upper portions 8 and 8′, or other materials, permeable or impermeable, which are compatible with the upper portion material. In accordance with a particularly preferred embodiment of the present invention, pieces of angle iron 26 are welded to the inside of the walls 24 of the box 22 so as to provide a continuous, inwardly projecting shelf which forms a lip defining the mouth l8 of the vacuum chamber 20. A continuous, elastomeric gasket 28 (e.g., silicone or fluoroelastomer rubber) is secured (e.g., glued) to the underside of the shelf 26 and is carried thereby for use in the casting of a plurality of moulds before replacement. The gasket 28 may be a simple O-ring, but will preferably have a rectangular cross-section for more effective sealing. The mould 6 will include a sealing surface on an upper surface thereof for engaging the underside of the gasket 28. This surface may be formed anywhere on the upper portion of a mould so long as a continuous surface can be provided. Preferably, however, the lower portion l0 of the mould 6 will include a continuous upstanding ridge 30 having an upper sealing surface 32 for engaging the elastomeric gasket 28 and compressing it against the shelf 26 when the mould 6 is secured to the vacuum box 22. The upstanding ridge 30 on the bottom mould portion l0 lies outside the porous upper shell portions 8 and 8′ so that the upper portions 8 and 8′, as well as the parting lines l2 and l2′, will confront the vacuum chamber 20 for the reasons set forth in European patent application No. 86307265.8. The mould 6 may be secured to the chamber 20 by means of inverted cups 34 which have self-tapping, female threads 35 on the inside surface thereof which are screwed onto upstanding mounting lugs 36 in the manner described in our copending European patent application Serial No. , filed concurrently herewith. The walls 24 of vacuum box 22 extend below the angle iron lip 26 of the vacuum chamber 20 so as to form a depending skirt portion 38 thereof. The skirt 38 depends sufficiently beneath the vacuum chamber 20 to surround the gasket 28 and at least the uppermost portion of the ridge 30 to shield the elastomeric gasket 28 from the radiant heat of the melt 4 in the pot 2 during casting. Preferably, a lower edge 40 of the skirt 38 will engage a shoulder 42 on the bottom portion l0 of the mould 6 and serves as a stop means for locating the mould 6 in the vacuum chamber 20. In this regard, the edge 40 abuts the shoulder 42 and prevents over-compression of the gasket 28 and generally ensures consistent positioning of each mould in the vacuum box 22.

Claims: 1. Apparatus for the vacuum countergravity casting of molten metal comprising: a mould (6) comprising a porous, gas-permeable upper shell (8,8′), at least in part defining a moulding cavity (l6,l6′), and a bottom-gated lower portion (l0) secured to said upper shell (8,8′) for admitting said molten metal into said cavity (l6,l6′) from an underlying pot (2) of said molten metal (4); and a vacuum box (22) defining a vacuum chamber (20) confronting said upper shell (8,8′) for evacuating said cavity (l6,l6′) through said shell (8,8′), characterised in that there is a sealing surface (32) provided on top of said mould (6) which is thermally remote from the molten metal (4) in said pot (2); said vacuum box (22) includes a peripheral wall (24) having a lip (26) on an inner side thereof which defines a mouth (l8) of said chamber (20); there is an elastomeric gasket (28) compressed between the underside of said lip (26) and said sealing surface (32) of said mould (6) for sealing said mould (6) to the mouth (l8) of said chamber (20); and there is a skirt (38) depending from said vacuum box (22) beneath said lip (26) so as to surround said gasket (28) and to shield it from heat radiating from said pot (2). 2. Apparatus for the vacuum countergravity casting of molten metal according to claim l, characterised in that the lower portion (l0) of the mould (6) has a continuous upstanding ridge (30) formed thereon surrounding said upper shell (8,8′) adjacent the periphery of the mould (6), which ridge (30) includes the sealing surface (32). 3. Apparatus for the vacuum countergravity casting of molten metal according to claim l or 2, characterised in that said elastomeric gasket (28) is secured to the underside of said lip (26), and has a rectangular cross-section.

Inventors: VOSS, KARL D.
Applicants: GENERAL MOTORS CORPORATION
Application year: 1986
Main classification: B22D18
IPC codes: B22D18