COUNTER-GRAVITY CASTING APPARATUS

Abstract: Immersion-type vacuum counter-gravity casting apparatus having a vacuum chamber (20) including spring means (54) for pressing upper and lower mould portions (8,10) sealingly together and/or resisting destructive inward flexure of the mould (6). A split vacuum chamber (20), including a floating lower skirt portion (72), avoids the creation of stress-concentration sites between the chamber (20) and the mould (6).

Description: This invention relates to apparatus for the vacuum, counter-gravity casting of metal in gas-permeable, shell-type moulds immersed in a pot of molten metal and, more particularly, to means for mounting the mould to the vacuum chamber so as to: eliminate the need to adhesively bond the mould portions (i.e., cope, drag, cheeks, etc.) together; resist destructive flexure of the mould during the application of the casting vacuum; and/or eliminate stress-concentration sites and provide a substantially uniform seal between the mould and the vacuum chamber. Background of the Invention The mould-immersion-type, vacuum, counter-gravity, shell mould casting process is particularly 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 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 U.S. patent 4,340,108 wherein the mould comprises a resin-bonded-sand shell having an upper cope portion and a lower drag portion sealingly mounted to the mouth of the vacuum chamber by means of spring clips. U.S. patent 4,340,108 discloses sealing 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. U.S. patent 4,632,171, upon which the preamble of claim 1 is based, discloses sealing the mould to the mouth of the vacuum chamber on top of the drag so that the parting line between the cope and drag falls within the vacuum chamber. U.S. patent 4,658,880 discloses mounting the mould to the vacuum chamber by means of a plurality of reciprocable and rotatable shafts having self-tapping threads on the lower ends thereof engaging mounting sites on top of the mould. Chandley, G.D. Automatic Countergravity Casting of Shell Moulds , Modern Casting, October 1983, pages 29-31, mounts round moulds to a round vacuum chamber having self-tapping threads which screw into the periphery of the mould. European patent application No.88312384.6 discloses mounting the mould to the vacuum chamber via a plurality of T-bar keepers engaging anchoring cavities in the mould. The aforesaid references all disclose rigid vacuum boxes and moulds whose upper and lower halves are glued together. The gluing process is expensive and time-consuming and elimination thereof would improve the efficiency and economics of the process. Moreover, when the aforesaid mould-chamber arrangements are used with moulds having more than about 2580.6 square centimetres (400 square inches) of mould confronting the vacuum chamber, there is a tendency for the moulds to bow or flex into the chamber when the casting vacuum is established therein unless they are made extra strong or thick. This flexure can destroy the mould either by cracking or fracturing the mould or occasionally causing implosion thereof into the chamber. Apparatus for the vacuum counter-gravity casting of molten metal according to the present invention is characterised by the features specified in the characterising portion of claim 1. It is the principal object of the present invention to provide an improved simple, self-adjusting apparatus for vacuum, counter-gravity casting, utilising unglued shell mould portions including means for resiliently biasing an upper mould portion into sealing engagement with a lower mould portion, resisting destructive flexure of the mould during casting and avoiding the creation of stress-concentration sites in the assembly. 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 contemplates mould-immersion-type countergravity casting apparatus of the type described above including spring means resiliently pressing the mould portions (i.e., cope, drag, cheeks) sealingly together (i.e., without adhesive). When large-area moulds are used, the spring means functions to resist destructive inward flexure of the moulds when the casting vacuum is established in the vacuum chamber, which function is served whether the mould parts are glued or not. More specifically, apparatus in accordance with the present invention includes: a mould which is adapted for immersion into an underlying pot of molten metal and which comprises a porous, gas-permeable, upper shell and a bottom-gated lower portion; a vacuum box defining a vacuum chamber confronting the upper shell for evacuating the mould through the shell, which box comprises (1) a ceiling overlying the mould, and (2) a skirt depending from the ceiling and surrounding the shell, which skirt has a peripheral edge on the underside thereof sealingly engaging the mould; means for mounting the mould in the mouth of the vacuum chamber; and spring means resiliently pressing the shell into sealing engagement with the lower mould portion and/or resisting destructive inward flexure of the mould when a vacuum is established in the vacuum box. The spring means provides the vacuum box with self-adjustability to compensate for process variations (e.g., variations in mould dimensions from one to the next) and will preferably be secured to a removable plate affixed to the inside of the chamber to minimize the number of possible vacuum leak sites. The vacuum box will preferably include a two-part skirt, i.e., a skirt which is horizontally split into an upper fixed portion carried by the mould/chamber transfer mechanism and a self-aligning, lower, floating portion. The upper and lower skirt portions are separated one from the other by a narrow (e. g., about 7.94 mm (5/16 inch)) gap which permits to and fro movement of the upper and lower portions relative to each other. Spring-containing retainers couple the upper and lower skirt portions together and serve to press the mould-sealing edge of the lower skirt portion down onto the mould so as to eliminate the creation of stress-concentration sites (i.e., high pressure points) and provide a substantially even/uniform pressure on the peripheral seal between the mould and lower skirt. Detailed Description of Specific Embodiments The invention may better be understood when considered in the light of the following detailed description of certain specific embodiments thereof which is given hereafter in conjunction with the accompanying drawings, in which: Figure 1 is a partially-sectioned, elevational view of one embodiment of a counter-gravity casting apparatus according to the present invention; Figure 2 is a partially-sectioned elevational view of another embodiment of a counter-gravity casting apparatus according to the present invention; Figure 3 is a partially-sectioned elevational view of still another embodiment of a counter-gravity casting apparatus according to the present invention; Figure 4 is a view in the direction 4-4 of Figure 3; Figure 5 is a partially-sectioned elevational view of still another embodiment of a counter-gravity casting apparatus according to the present invention; Figure 6 is a view in the direction 6-6 of Figure 5; Figure 7 is an enlargement of a portion of a vacuum chamber of Figure 3; Figure 8 is an enlargement of a seal clamping bar of Figure 7; and Figure 9 is a portion of a vacuum chamber like that of Figure 3 showing a preferred embodiment of a cope-biasing spring. Figure 1 depicts a pot 2 of metal melt 4 which is to be drawn up into a mould 6 comprising a gas-permeable upper shell portion 8 and a lower portion 10 joined at a parting line 12 and defining a moulding cavity 14 therebetween. The lower portion 10 includes a plurality of ingates 16 in the underside thereof for admitting melt 4 to the mould cavity 14 when it is evacuated through the shell portion 8. The lower portion 10 of the mould 6 is sealed to a mouth 18 of a vacuum chamber 20 (i.e., defined by vacuum box 22) via a compressible seal 24 (e.g., high-temperature rubber or ceramic rope) affixed to a lower peripheral edge of a depending skirt 21 of the box 22. The vacuum chamber 20 encompasses the upper shell portion 8 of the mould 6 and communicates with a vacuum source (not shown) via conduit 26. The upper shell portion 8 of the mould 6 comprises a gas-permeable material (e.g., resin-bonded sand or ceramic) which permits gases to be withdrawn from the casting cavity 14 therethrough when a vacuum is established in the chamber 20. The lower mould portion 10 of the mould 6 may conveniently comprise either the same material as the upper shell portion 8 or other materials, permeable or impermeable, which are compatible with the upper portion material. The lower mould portion 10 includes an upstanding levee 26 surrounding the seal 24 and isolating it from the melt 4 as described in copending European patent application No.0301693. The lower mould portion 10 includes a plurality of anchoring sites 28 engaged by T-bar keepers 30 of the type described in the aforesaid European patent application No.88312384.6 which relates to such means for mounting the mould 6 to the vacuum box 22. As described in that application, the lower portion 10 of the mould 6 includes a plurality of anchoring cavities 32 adapted to receive T-bar keepers 30 via slots 34 in shelves 40 overlying the anchoring cavities 32. A 90° rotation of the T-bar carrying shafts 36 (e.g., by air motors 38) causes the T-bar keepers 30 to engage the underside of the respective shelves 40 overhanging the cavities 30 to secure the mould 6 to the box 22. Other mounting means such as disclosed in the other references (supra) would, of course, also be acceptable. The upper shell portion 8 is pressed into sealing engagement with the lower mould portion 10 (i.e., at the parting line 12) by means of a plurality of plungers 42. Feet 44 on the ends of the plungers 42 distribute the force of the plungers 42 more widely across the top of the shell portion 8 to prevent penetration/puncture thereof by the ends of the plungers 42. Pneumatic springs 46 bias the plungers 42 downwardly to resiliently press the shell portion 8 against the lower mould portion 10 as the mould 6 is being positioned in the mouth 18 of the box 22. Schrader valves 48 on the air springs 46 permit varying of the pressure in the springs 46 as needed to apply sufficient force to press the upper shell portion 8 into sealing engagement with the lower mould portion 10, and, as needed, to prevent destructive inward flexure of the mould 6 when the casting vacuum is established. The force applied by the plungers 42, however, will not be so great as to overpower and damage the anchoring sites 28, dislodge the mould 6 from the mouth 18 of the box 22, or break the seal formed thereat. In accordance with another embodiment of the present invention, Figure 2 depicts a counter-gravity casting apparatus similar to that of Figure 1 but differing therefrom with respect to the nature of the spring means used to press the upper shell portion 8 against the lower mould portion 10. The structural elements of the apparatus of Figure 2 which are common to the structural elements of the apparatus of Figure 1 have the same numerical designation. The apparatus of Figure 2 differs from that of Figure 1 in that the vacuum box 22 has a removable ceiling 50 which permits ready changeover from one size vacuum box to the next by merely bolting on differently-dimensioned skirts 21. Moreover, the separable ceiling 50 provides topside access to the vacuum chamber 20 for removal of a carrier plate 52 used to support and carry spring means 54 totally within the confines of the box 22. More specifically, the carrier plate 52 is bolted to ears 56 welded to the inside of the skirt 21 of the box 22. The plate 52 may include apertures 58, as necessary, to ensure that the entire chamber 20, on both sides of the plate 52, is maintained at substantially the same sub-atmospheric pressure during casting and to permit gasses generated during the moulding process to exhaust from the chamber 20 via the conduit 26. In this embodiment, each spring means 54 comprises a shaft 59 within a coil spring 68 and having a head 60 on the upper end thereof and an external thread 62 on the lower end thereof. The shaft 59 slides through an opening 64 in the plate 52 with the head 60 serving as a stop to prevent the shaft 59 from falling or being pushed out of the opening 64. A foot 66 having an internal thread (not shown) is screwed onto the thread 62 and may be used to fine-tune the length of the shaft 59 and the force exerted by the coil spring 68 compressed between the foot 66 and the underside of the plate 52 as shown. Before the mould 6 is assembled to the box 22, the spring means 54 will hang from the plate 52 by engagement of the heads 60 therewith. When the mould 6 is positioned in the mouth 18 of the box 22, the upper shell 8 pushes up on the lower end of the spring means 54 (i.e., collars 66) causing compression of the coil springs 68 and upward unseating of the heads 60 from the top of the plate 52. In this position, the compressed springs 68 push back on the upper shell portion 8 with sufficient force to cause it to seat and seal on top of the lower mould member 10 and to resist the tendency of the mould 6 to flex or bow inwardly when a vacuum is established in the chamber 20. The force supplied by the spring 68 will, however, not be so great as to break the mounting sites 28, disrupt the seal formed at the mouth 18 of the box 22 or otherwise dislodge the mould 6 from the box 22. The embodiments shown in Figures 3 and 4 are similar to that shown in Figure 2 but contain additional features described hereafter relating to another important and preferred feature of the invention. More specifically, the skirt depending from the ceiling 50 of the vacuum box 22 is horizontally separated into an upper skirt portion 70 and a lower skirt portion 72 separated one from the other by a gap 74. The gap 74 will typically be about 7.94 mm (5/16 inch) wide. As best shown in Figure 7, a 50.8 mm (two inch) wide flexible sealing member 76 co-extensive with the gap 74 is secured to the upper and lower skirt portions 70 and 72, respectively, so as to cover the gap 74 and thereby maintain the integrity of the vacuum chamber 20 when the vacuum is established therein yet permit the lower skirt portion 72 to float sufficiently to level or true itself with respect to the mould 6 even when the horizontal plane of the mould is not perfectly parallel to the sealing edge of the vacuum box 22. The flexible seal 76 comprises a 15.24 mm (0.60 inch) thick gas-impermeable Fiberglas-filled silicone rubber material commonly used for conveyor belts and provided by the F. B. Wright Co., U.S.A. as Material No.GP 207-100-MC-2-108. This seal material was found to be particularly effective in resisting inward ballooning and rupturing when the vacuum is established in the chamber yet still be flexible enough for the intended purpose. The seal 76 is attached to the upper and lower skirt portions 70 and 72, respectively, by a pair of continuous bar clamps 77 bolted to the upper and lower skirt portions at a plurality of locations. As best shown in Figure 8, the bar clamps 77 each include a base portion 79 for bolting to the skirt and a leg portion 81 extending from the base portion 79 to define a continuous recess 83 therebetween for engaging and pressing the seal 76 tightly against the inside wall of the skirt. The inside face 85 of the leg 81 lies at an acute angle (preferably about 85°) to the face 87 of the base 79 to provide a sharp edge 89 which bites into the seal 76 to firmly hold the seal 76 in place. A sheet metal shield 78 is secured along its bottom edge 80 to the lower skirt portion 72 and extends upwardly and over the seal 76 to protect it from physical and/or thermal damage (e.g., metal spatter). The upper edge 82 of the shield 78 is unattached and is free to slide along the inside surface of the upper skirt portion 70 as the gap 74 opens and closes in the manner described hereinafter. The upper and lower skirt portions 70 and 72, respectively, are held together by a retaining means 84 which permits the lower portion 72 to float somewhat independently of the upper portion 70 yet prevents it from so separating from the upper portion 70 as to damage the seal 76. More specifically, the retainer means 84 includes an upper bracket 86 secured (e.g., welded) to the upper skirt member 70 and a lower bracket 88 welded to the lower skirt portion 72. A bolt 90 extends loosely through the brackets 86 and 88 so as to permit relative movement between the bolt and the brackets. A coil compression spring 92 surrounds the bolt 90. The combination of the gap 74, retainer means 84 and flexible seal 76 permits the lower skirt portion 72 to float relative to the upper skirt portions 70 to better receive the mould 6 without damaging it such as could occur if pressure points or stress sites were otherwise created. The springs 92 press the lower skirt portion 72 down against a sealing surface 94 on top of the lower mould portion 10 so as to provide a substantially uniform sealing pressure therebetween regardless of any unlevel or unplumb condition existing between the mould 6 and the box 22. In the embodiment shown in Figures 3 and 4, the mould 6 is supported on hangers 96 having L-shaped hooks 98 which carry the mould 6 from a loading station to the casting station shown in Figure 3. In operation, the mould 6 is first placed on the hangers 96 (i.e., at the loading station) and the vacuum box 22 lowered to engage a stop located so that the lower skirt portion 72 touches/engages the mould 6 with substantially no compression of the springs 68 or 90. The thusly mated mould 6 and box 22 are then transferred to the casting station and immersed in the melt 4. At that time, the buoyant forces of the melt cause the mould 6 to float off of the hooks 98, narrow the gap 74, and compress the springs 68 and 90 until equilibrium is established. Finally, when the vacuum is established in the chamber 20, the mould 6 is drawn further off the hooks 98 and up into the box 22 further closing the gap 74 and compressing the springs 68 and 90. The unique features of this, the preferred embodiment of the present invention, provide a self-adjusting system which accommodates wide process variations without stressing the moulds to the point of breakage. Figures 5 and 6 depict still another embodiment of present invention and, more specifically, show a mould 100 having an upper portion 102 resiliently pressed against a lower portion 104 by means of coil springs 106 surrounding shafts 108 used to carry T-bar keepers 110. In this regard, a washer 112 adapted to slide axially along the shaft 108 engages the top surface of the upper portion 102 surrounding a slot 114 in the upper portion 102 through which the T-bar keepers 110 passes to access an anchoring cavity 116 formed in the lower mould portion 104. In operation (i.e., at the loading station) the vacuum box 22 descends upon the mould 100 until seals 24 sealingly engage the upper surface of the lower mould portion 104. Thereafter, an air cylinder 118 lowers the T-bar locking mechanism through the slots 114 until the T-bar keepers 110 are fully within the anchoring cavities 116. At that time, air motors 120 rotate the T-bar keepers to secure the mould 100 to the vacuum box 22. At the same time, the upper surface of the upper mould portion 102 engages the washers 112 forcing them upwardly along the shafts 108 and compressing the springs 106 which resiliently press the upper portion 102 down against the lower mould portion 104. Figure 9 depicts a preferred embodiment of spring-biased plunger pressing the cope to the drag. In this embodiment a spring retainer plate 122 is spaced from a roof 124 of the vacuum chamber by a plurality of spacers 126 and a plunger shaft 128 passes therethrough as described above in conjunction with Figure 3. In this embodiment, however, the shaft 128 includes a longer thread 130 on the lower end thereof for receiving a threaded spring compression adjusting collar 132 as well as a threaded foot 134 so as to provide independent adjustment of the spring compression and the shaft length as may be needed for fine-tuning the system.

Claims: Apparatus for the vacuum counter-gravity casting of molten metal (4) comprising: a frangible mould (6;100) comprising a porous gas-permeable upper shell (8;102) at least in part defining a moulding cavity (14) and a lower portion (10;104) adapted for immersion into a pot (2) of said molten metal (4) underlying said mould (6;100) when filling said cavity (14) with said molten metal (4) and emersion from said pot (2) after said filling, said lower portion (10;104) including at least one gate (16) in the underside thereof for admitting said molten metal into said cavity (14) upon evacuation of said cavity (14); a vacuum box (22) defining a vacuum chamber (20) confronting said upper shell (8;102) for evacuating said cavity (14) through said shell (8;102), said box (22) including a peripheral edge on the underside thereof defining a mouth (18) receiving and sealingly engaging said lower portion (10;104); and means (30;96,98;110) for mounting said mould (6;100) to said mouth (18) with said lower portion (10;104) in sealing engagement with said peripheral edge; characterised in that the apparatus includes spring means (42,46;54;106) in said chamber (20) which engages said upper shell (8;102) when said mould (6,100) is positioned in said mouth (18), pressing said upper shell (8;102) into sealing engagement with said lower portion (10;104) and resisting external force on said mould (6;100) tending to push said mould (6,100) into said chamber (20). Apparatus according to claim 1, characterised in that said spring means comprises a plunger (42;59;108;128) engaging the top of said upper shell (8;102). Apparatus according to claim 2, characterised in that said spring means comprises a coil spring (68;106) surrounding said plunger (59;108). Apparatus according to claim 3, characterised in that said spring means includes adjusting means (66,132) adjustable along the length of said plunger (59;128) to adjust the force applied by said coil spring (68). Apparatus according to claim 4, characterised in that said adjusting means comprises a threaded collar (66;132) engaging a screw-thread on said plunger (59;128). Apparatus according to claim 1, characterised in that said spring means comprises a pneumatic spring (46). Apparatus according to claim 1, characterised in that the apparatus includes a mounting plate (52) supporting said spring means (54) and secured to said box (22) within said chamber (20) overlying said mould (6). Apparatus according to claim 1, characterised in that said vacuum box (22) comprises a ceiling (50) overlying said mould (6) and a skirt depending from said ceiling (50) and surrounding said shell (8), said skirt having a first portion (70) engaging said ceiling (50) and a second portion (72) underlying said first portion (70), said second skirt portion (72) being movably spaced from said first skirt portion (70) by a gap (74) and including a peripheral edge on the underside thereof defining said mouth (18); and a substantially gas-impermeable, flexible seal (76) engaging said first and second skirt portions (70,72) substantially co-extensively with said gap (74) to permit relative movement between said first and second skirt portions (70,72) whilst maintaining the integrity of said vacuum chamber (20); said spring means (54) resiliently pressing on said shell (8) during the application of vacuum to said chamber (20) to press said shell (8) into said sealing engagement with said lower mould portion (10) and to permit relative motion between said mould (6) and said upper skirt portion (70) when said mould (6) is immersed in said molten metal (4). Apparatus according to claim 8, characterised in that said flexible seal (76) comprises Fiberglas-filled silicone rubber. Apparatus according to claim 8, characterised in that the apparatus includes clamping means (77) engaging the longitudinal edges of said flexible seal (76) substantially continuously about said seal (76) to press said edges securely against said skirt portions (70,72) on opposite sides of said gap (74), said clamping means comprising a pair of elongated bars (77), each elongated bar (77) having an elongated recess (83) therein receiving a respective edge of the seal (76) and being defined by walls (85,87) extending at an acute angle one to the other, there being an elongated pressure ridge portion (89) provided along one side (85) of said recess (83) for biting into said seal (76) so as to press it tightly against said respective skirt portion and to trap said respective edge in said recess (83). Apparatus according to claim 8, characterised in that said flexible seal (76) lies inside said chamber (20), and a shield (78) secured to one of said skirt portions (70) overlies said seal (76) to protect said seal (76) from damage. Apparatus according to claim 11, characterised in that said apparatus includes retainer means (84) coupling said first and said second skirt portions (70,72) together in substantially aligned relation one to the other across said gap (74), said retainer means (84) being adapted to permit relative movement between said skirt portions (70,72). Apparatus according to claim 12, characterised in that said retainer means (84) includes spring means (92) for resiliently pressing said second skirt portion (72) into sealing engagement with said lower mould portion (10) substantially uniformly along said peripheral edge. Apparatus according to claim 13, characterised in that said spring means comprises a plurality of springs (92) distributed about the periphery of said vacuum box (22).

Inventors: HAFER, PAUL H., HANSON, BRADLEY W., PORTER, JEFFREY D., SMITH, JAMES, JR.
Applicants: GENERAL MOTORS CORPORATION
Application year: 1989
Main classification: B22D18
IPC codes: C09K3, B22D18