COUNTERGRAVITY CASTING APPARATUS

Abstract: Apparatus for the vacuum, countergravity casting of metal in shell moulds (6) including a gas-permeable shell mould (6) secured to the mouth (l8) of a vacuum box (22) by means of a plurality of threadable mounting sites (34) on top of the mould engaged by self-tapping threads (44) on the ends of rotatable shafts (40) reciprocally slidable through a ceiling (38) of the vacuum box (22). The mounting sites may comprise upstanding lugs (34) on top of the mould (6).

Description: This invention relates to apparatus for the vacuum, countergravity casting of metal in gas-permeable, shell moulds and, more particularly, to means for mounting the mould to the vacuum chamber used therewith. 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 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 and attached to the vacuum chamber by means of spring clips which engage a peripheral abutment on the outside of the vacuum chamber. 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. Spring-biased bolts are used which engage the underside of the drag and extend along the outside of the vacuum chamber to secure the mould to the vacuum chamber. The heads of the bolts are immersed in the melt and accordingly have a very short useful life. Finally, Chandley G.D., Automatic Counter Gravity Casting of Shell Moulds , Modern Casting, October l983, pages 29-3l, describes a technique for mounting round moulds to a round vacuum chamber wherein the inside surface of the vacuum chamber includes self-tapping threads which screw into the periphery of the round mould. The latter technique has been restricted to relatively small moulds and cannot be used with moulds which are rectangular or have other than a round exterior. It is an object of the present invention to provide apparatus for the vacuum, countergravity casting of shell moulds including improved means for automatically mounting the shell mould to the mouth of the vacuum box without deterioration of the mounting means and regardless of the shape or size of the mould and the vacuum chamber. 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 vacuum box defining a chamber confronting the upper shell for evacuating the mould through the shell, which box comprises a peripheral wall having a lip on the underside thereof for sealingly engaging the mould and a ceiling overlying the mould; a plurality of threadable mounting sites on top of the mould; and retainer means reciprocably slidable through the ceiling of the box, said retainer means comprising a plurality of shafts extending sealingly through the ceiling, self-tapping threads on the lower end of each shaft engaging a mounting site registered therewith, and a means for rotating each shaft so as to screw the threads into engagement/disengagement with the mounting site as is appropriate for mounting and demounting the mould to the vacuum chamber. According to one embodiment of the invention the mounting sites each comprise an upstanding lug and the lower end of the shaft comprises an inverted cup having female threads on the inside thereof for threading onto the outside surface of the lugs. In another embodiment of the invention, the lower end of each shaft is provided with a male threaded tip for screwing into the mounting site. In this latter embodiment, the mounting site may be an upstanding lug similar to that of the first embodiment, or may simply be a location on the main body of the mould where the threaded tip can burrow into the mould without upsetting the moulding cavity. A rotator (e.g., air motor) on the other end of the shaft causes the shaft to rotate in the desired direction for screwing the threads onto/into the mounting sites so as to draw the mould up into sealing engagement with the mouth of the vacuum chamber. Detailed Description of Specific Embodiment The invention may better be understood when considered in the light of the following detailed description of certain specific embodiments thereof which are given hereafter in conjunction with the accompanying drawings, in which: Figures l, 2 and 3 are side, sectioned views through different embodiments of a vacuum, countergravity metal casting apparatus in accordance with the present invention. Figures l, 2 and 3 differ one from the other only with respect to the precise nature of the mounting sites used to anchor the mould to the vacuum chamber. Accordingly, like reference numerals are used for like parts in all three Figures and different reference numerals are used only where the Figures differ from each other. The various Figures show a pot 2 of metal melt 4 which is to be drawn up into the 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 the 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 produced 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. 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 or 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 for effecting a seal between the mouth l8 of the vacuum chamber 20 and the mould 6. The lower portion l0 of the mould 6 includes 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. In accordance with one embodiment of the present invention, upstanding mounting lugs 34 are provided on top of the upper portions 8, 8′ of the mould 6, which lugs 34 are adapted to be threaded by self-tapping threads on the ends of anchoring means 36 which extend through a ceiling 38 of the vacuum chamber 20. In the particular version of that embodiment shown in Figure l, the anchoring means 36 comprises a rotatable shaft 40 having an inverted cup 42 on the lower end thereof which, in turn, has self-tapping, female threads 44 on the inside surface thereof. Upon rotation of the shaft 40, the threads are screwed into the outside surface of the respective upstanding mounting lug 34. In another version of that embodiment (shown in Figure 2), the lower end of the shaft 40 is provided with a tip 46 having male, self-tapping threads on the outside thereof which, upon rotation of the shaft 40, screw into the centre of a respective stud 48. The stud 48 will preferably be pre-drilled to provide an undersized socket 49 for receiving the respective tip 46 therein and thereby reduce the risk of splitting the stud apart during engagement with the tip 46. The shafts 40, for both embodiments, have motors 50 on the opposite ends thereof for rotating the shafts 40 so as to engage and disengage the threads from the mounting lugs. The shaft 40 of each anchoring means 36 is slidable up or down through the centre of bearings 52 carried by a mounting bracket 54 and through a rubber (e.g., silicone) sealing grommet 56. The embodiment shown in Figure 3 is similar to that of Figure 2 except that the mounting studs 48 are eliminated and sockets 49′ are provided directly into the main body of the mould 6 through a top surface 58 thereof. In operation, the mouth of the vacuum chamber 20 is lowered down onto the mould 6. Thereafter, the anchoring means 36 are caused, by any convenient means (not shown), to descend into engagement with the respective lugs 34 or studs 48, registered therewith, and the shafts 40 rotated (e.g., clockwise) to screw the threads 44 or 46 onto/into the lugs 34 or studs 48 respectively and thereby draw the mould 6 up tightly against the gasket 28. After casting, the shafts 40 are rotated in the opposite direction (e.g., counterclockwise) to release the mould 6. It will be appreciated from the foregoing disclosure that the present invention has the advantages that the anchoring means used to fasten the mould to the vacuum chamber are all located within the vacuum chamber out of contact with, and protected from, the melt, and the mould can be readily attached or detached from the vacuum chamber using automation.

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 plurality of threadable mounting sites (34,48) on top of said mould (6), said vacuum box (22) has a ceiling (38) overlying said mould (6), and there are means (36) reciprocally slidable through said ceiling (38) for engaging each of said sites (34,48) so as to anchor said mould (6) in said chamber (20), each of said means comprising a shaft (40) extending through said ceiling (38), a self-tapping thread (44,46) formed on the lower end of said shaft (40) for engaging a respective site (34,48) registered therewith, and rotator means (50) operatively associated with the shaft for screwing said thread (44,46) into engagement/disengagement with said respective site (34,48) so as to mount/demount said mould (6) to/from said vacuum box (22). 2. Apparatus for the vacuum countergravity casting of molten metal according to claim l, characterised in that each of said sites (34) comprises an integral, threadable mounting lug, and the lower end of each shaft (40) comprises an inverted cup (42) with the self-tapping thread (44) being formed as a female thread on the interior of said cup (42) for screwing onto the outside surface of said mounting lug (34). 3. Apparatus for the vacuum countergravity casting of molten metal according to claim l, characterised in that each of said sites (48) comprises an integral, threadable mounting stud pre-drilled to provide a socket (49), and the self-tapping thread (46) is a male thread for screwing internally into said socket (49). 4. Apparatus for the vacuum countergravity casting of molten metal according to claim l, characterised in that each of said sites comprises a socket (49′) formed in a top surface (58) of the mould (6), and the self-tapping thread (46) is a male thread for screwing internally into said socket (49′).

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