COUNTERGRAVITY CASTING MOULD AND CORE ASSEMBLY

Abstract: Apparatus for a countergravity, shell-mould casting process comprising a gas-permeable shell mould (6) seated in the mouth (l8) of a vacuum chamber (20) and a hollow, expendable, gas-permeable, thermally degradable, core (24) disposed with a moulding cavity (l6) for engulfment and retention by the metal being cast. The core (24) defines a central evacuation cavity (28) and an unobstructed vent (36,36′) for exhausting the evacuation cavity (28) into the vacuum chamber (20) so that the pressure in the evacuation cavity (28)is substantially equal to that in the vacuum chamber (20) during casting.

Description: This invention relates to countergravity casting of metal in gas-permeable, shell moulds and more particularly to thermally-degradable, retained, expendable cores therefor. Background of the Invention The countergravity, shell mould, casting process is particularly useful in the making of thin-wall castings and involves: sealing a bottom-gated shell mould, having a gas-permeable upper portion, (e.g., cope) to the mouth of a vacuum chamber such that the chamber encompasses 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 cope and drag portions defining a moulding cavity therebetween. Many castings made by such a process require the use of an expendable, retained core disposed within the mould cavity to shape the inside of the casting. Such cores are engulfed by the melt, initially retained within the casting and finally removed therefrom as, for example, by disintegration. It is known to use hollow retained cores to reduce the amount of core material and to facilitate core removal. Retained cores typically have a mounting extension on at least one end thereof which is anchored to the mould shell (i.e., usually at the parting line between the shell halves) to position the core in the moulding cavity and support it against movement therein as the melt flows about it. Heretofore, the mounting extension has been simply buried deep within the material forming the mould shells, and, for thermally stable core materials (e.g., quartz), this is an acceptable way to mount the core. Such materials, however, are quite expensive especially in complicated shapes. Less expensive core materials such as resin-bonded-sand (e.g., hot-box, cold-box, or shell), or similar material, on the other hand, can be formed into virtually any core shape desired and hence give the mould maker considerable flexibility. However, resin-bonded-sand core materials are thermally-degradable in that the resin binder breaks down to form gases under the heat of the melt. With respect to such thermally-degradable, retained cores, it has been found that the gases generated by the breakdown of the binder during casting are trapped by the surrounding metal and hence cannot escape the moulding cavity through the walls of the gas-permeable shell walls. Instead, these trapped gases tend to become detrimentally occluded (e.g., as internal voids or surface pits) in the casting. It is therefore an object of the present invention to provide an improved countergravity casting apparatus of the above-described type which is so constructed and arranged as to vent the breakdown gases generated by thermally-degradable, retained cores engulfed by metal within the moulding cavity and thereby avoid occlusion of the gases in the finished casting. 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 present invention comprehends a countergravity, shell mould casting apparatus including essentially: a vacuum chamber; a shell mould having a gas-permeable upper portion (e.g., cope) secured to a bottom-gated lower portion (e.g., drag) and sealed in the mouth of the vacuum chamber; and a hollow, thermally-degradable, gas-permeable, expendable, retained core having an internal evacuation cavity which is vented to the vacuum chamber via a substantially unobstructed gas-flow passage. More specifically, the thermally-degradable core material (e.g., resin-bonded-sand) forms an appropriately shaped shell defining an internal evacuation cavity. The core has a mounting extension on at least one end thereof and the evacuation cavity is unobstructedly vented to the vacuum chamber via a passage through the extension such that the pressure in the evacuation cavity during casting is as near to the reduced pressure in the vacuum chamber as is possible. As a result, any gases formed by the thermal degradation of the core material by the surrounding melt are immediately sucked through the gas-permeable core shell into the evacuation cavity and exhausted to the vacuum chamber thereby preventing occlusion thereof in the casting. The evacuation cavity of the core will preferably communicate with the vacuum chamber as directly as possible, as by bringing the core extension, and hence the vent passage therethrough, through the mould shell to the surface of the mould in the vacuum chamber. Where this is not possible, the evacuation cavity may be vented indirectly by an opening in the shell formed as, for example, by boring a supplemental passage through the mould shell into registry with the passage to the evacuation cavity through the core extension. Boring vent passages requires precise location of the part to ensure that the bore accurately meets the passage through the extension, and is thus an additional processing step. Hence direct venting is preferred wherever the part design will permit. Detailed Description of a Specific Embodiment of the Invention The present invention may better be understood when considered in the light of the following detailed description of certain specific embodiments thereof which are described hereafter in conjunction with the accompanying drawings, in which: Figures l and 2 are sectioned, side views of countergravity, shell mould casting apparatus in accordance with the present invention. While Figures l and 2 disclose different embodiments of the present invention, they are best described using the same reference numerals for like parts, where applicable. In this regard, the embodiments shown in Figures l and 2 differ only with respect to how (i.e., indirectly or directly, respectively) the hollow cores are vented to the vacuum chamber. More specifically, Figures l and 2 disclose a pot 2 of metal melt 4 which is to be drawn up into the mould 6. The mould 6 includes a first portion 8 joined (e.g., glued) to a second, lower portion l0 along a parting line l2 and define therebetween a moulding cavity l6. The lower portion l0 includes a plurality of ingates l4 on the underside thereof for supplying melt to the mould cavity l6. The lower portion l0 of the mould 6 is sealed to a mouth l8 of the vacuum chamber 20 such that the upper portion 8 is encompassed by the chamber 20. The vacuum chamber 20 is communicated 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 or evacuated from the casting cavity l6 when a vacuum is drawn in the chamber 20. The lower portion l0 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. An expendable, retained hollow core 24 comprising a gas-permeable, thermally-degradable shell 26 defining an internal evacuation cavity 28 is positioned substantially centrally within the casting cavity l6 of the mould 6 and is completely engulfed by the melt during filling. The core 24 includes extensions 30 and 30′ on the opposite ends thereof which are secured (i.e., by glue 32) to the mould 6 in recesses previously moulded into the upper and lower portions 8 and l0 at the parting line l2. Passages 34 and 34′ through the centres of the extensions 30 and 30′ respectively communicate the evacuation cavity 28 with outboard ends 3l and 3l′ of the extensions 30 and 30′. In some instances depending on the design of the casting, a single core extension may be sufficient to locate and immovably anchor the core in the moulding cavity. Indeed some castings may permit the use of only one core extension in order to meet design requirements. In the embodiment shown in Figure l, the ends 3l and 3l′ of the extensions 30 and 30′ are buried deep within the mould and hence the passages 34 and 34′ would normally be obstructed by the mould material if it were not for the present invention. In accordance with this invention, bores 36 and 36′ are provided through the upper portion 8 of the mould 6 so as to indirectly provide unobstructed communication between the evacuation cavity 28 and the vacuum chamber 20 via the passages 34 and 34′. In the embodiment shown in Figure 2, the upper shell portion 8 of the mould 6 is formed so as to be peripherally smaller than the mouth l8 of the chamber 20. In this embodiment, the core extensions 30 and 30′ extend completely through the upper shell 8 so as to exit on the outside surface 38 and 38′ thereof. This permits the passages 34 and 34′ to vent the evacuation cavity 28 directly to the vacuum chamber 20. In this embodiment, recesses 40 and 40′, which are formed in the lower portion of the mould l0 to receive extensions 30 and 30′, are elongated sufficiently so as not to block the passages 34 and 34′ and therefore ensure that there are no obstructions to interfere with gas flow out of the evacuation cavity 28. Needless to say, the hollow cores in accordance with the present invention need not necessarily lie horizontally in the moulding cavity but may assume a variety of orientations (e.g., vertical, or oblique) and may be affixed to the mould at many locations (e.g., depend from the top) without departing from the essence of the present invention. Hence, while the invention has been disclosed primarily in terms of two 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 the countergravity casting of molten metal comprising: a mould (6) comprising a porous, gas-permeable upper shell (8) defining a moulding cavity (l6) and a lower portion (l0) secured to said upper shell (8), said lower portion (l0) having at least one gate (l4) on the underside thereof for admitting said molten metal to said moulding cavity (l6) from an underlying pot (2) thereof; a vacuum chamber (20) overlying said upper shell (8) and sealingly engaging said mould (6) for withdrawing gases from said moulding cavity (l6) substantially uniformly through said shell (8) and such as to suck said molten metal up into said cavity (l6) via said one gate (l4); and a discrete, hollow, expendable core (24) disposed within said moulding cavity (l6) to shape said metal thereabout, said core (24) being destined for engulfment and temporary retention by said metal, characterised in that said core (24) comprises a porous, gas-permeable, thermally-degradable shell (26) defining an evacuation cavity (28) for withdrawing gases from said moulding cavity (l6) which are formed upon degradation of said core (24) whilst engulfed by said metal during casting; an extension (30,30′) on said core (24), said extension (30,30′) projecting from at least one end thereof and sealingly engaging said mould (6); and an unobstructed gas flow passage (34,34′) through said extension (30,30′) connecting said evacuation cavity (28) and said vacuum chamber (20) for establishing a pressure in said evacuation cavity (28) during casting which is substantially equal to the pressure in said vacuum chamber (28);so that gases generated by the thermal degradation of said core (24) are withdrawn from said moulding cavity (l6) into said vacuum chamber (20) via said evacuation cavity (28) and passage (30,30′), thereby preventing the occlusion thereof in the metal casting. 2. Apparatus for the countergravity casting of molten metal according to claim l, characterised in that there is a bore (36,36′) in said upper shell (8) connecting said passage (34,34′) and said vacuum chamber (20) for maintaining a pressure in said evacuation cavity (28) during casting which is substantially equal to the pressure in said vacuum chamber (20).

Inventors: DATTE, MARK A., RUFF, GARY F., VOSS, KARL D.
Applicants: GENERAL MOTORS CORPORATION
Application year: 1986
Main classification: B22D18
IPC codes: B22D18, B22C9