EP0474078B1 - Countergravity casting using particulate supported thin walled investment shell mold - Google Patents
Countergravity casting using particulate supported thin walled investment shell mold Download PDFInfo
- Publication number
- EP0474078B1 EP0474078B1 EP19910114237 EP91114237A EP0474078B1 EP 0474078 B1 EP0474078 B1 EP 0474078B1 EP 19910114237 EP19910114237 EP 19910114237 EP 91114237 A EP91114237 A EP 91114237A EP 0474078 B1 EP0474078 B1 EP 0474078B1
- Authority
- EP
- European Patent Office
- Prior art keywords
- shell
- support media
- molten metal
- mold cavity
- housing
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
Links
- 238000005266 casting Methods 0.000 title claims description 55
- 229910052751 metal Inorganic materials 0.000 claims description 51
- 239000002184 metal Substances 0.000 claims description 51
- 238000000034 method Methods 0.000 claims description 25
- 230000035699 permeability Effects 0.000 claims description 16
- 239000000463 material Substances 0.000 claims description 13
- 238000010438 heat treatment Methods 0.000 claims description 7
- 239000004576 sand Substances 0.000 claims description 3
- XSQUKJJJFZCRTK-UHFFFAOYSA-N Urea Chemical compound NC(N)=O XSQUKJJJFZCRTK-UHFFFAOYSA-N 0.000 claims description 2
- 239000004202 carbamide Substances 0.000 claims description 2
- 238000003825 pressing Methods 0.000 claims 3
- 230000035882 stress Effects 0.000 description 24
- 239000007789 gas Substances 0.000 description 22
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 description 12
- 238000007598 dipping method Methods 0.000 description 8
- 239000002002 slurry Substances 0.000 description 8
- 230000008901 benefit Effects 0.000 description 6
- 230000001965 increasing effect Effects 0.000 description 6
- 239000011819 refractory material Substances 0.000 description 6
- 238000005336 cracking Methods 0.000 description 5
- 229910052845 zircon Inorganic materials 0.000 description 5
- GFQYVLUOOAAOGM-UHFFFAOYSA-N zirconium(iv) silicate Chemical compound [Zr+4].[O-][Si]([O-])([O-])[O-] GFQYVLUOOAAOGM-UHFFFAOYSA-N 0.000 description 5
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 4
- 239000011230 binding agent Substances 0.000 description 4
- KZHJGOXRZJKJNY-UHFFFAOYSA-N dioxosilane;oxo(oxoalumanyloxy)alumane Chemical compound O=[Si]=O.O=[Si]=O.O=[Al]O[Al]=O.O=[Al]O[Al]=O.O=[Al]O[Al]=O KZHJGOXRZJKJNY-UHFFFAOYSA-N 0.000 description 4
- 238000001035 drying Methods 0.000 description 4
- 239000005350 fused silica glass Substances 0.000 description 4
- 229910052863 mullite Inorganic materials 0.000 description 4
- 238000001816 cooling Methods 0.000 description 3
- 239000012528 membrane Substances 0.000 description 3
- 230000002093 peripheral effect Effects 0.000 description 3
- 239000000843 powder Substances 0.000 description 3
- 230000009467 reduction Effects 0.000 description 3
- KFZMGEQAYNKOFK-UHFFFAOYSA-N Isopropanol Chemical compound CC(C)O KFZMGEQAYNKOFK-UHFFFAOYSA-N 0.000 description 2
- BOTDANWDWHJENH-UHFFFAOYSA-N Tetraethyl orthosilicate Chemical compound CCO[Si](OCC)(OCC)OCC BOTDANWDWHJENH-UHFFFAOYSA-N 0.000 description 2
- PNEYBMLMFCGWSK-UHFFFAOYSA-N aluminium oxide Inorganic materials [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 description 2
- 230000008859 change Effects 0.000 description 2
- 239000008119 colloidal silica Substances 0.000 description 2
- 230000007547 defect Effects 0.000 description 2
- 238000004519 manufacturing process Methods 0.000 description 2
- 239000000155 melt Substances 0.000 description 2
- 238000005058 metal casting Methods 0.000 description 2
- 229910052757 nitrogen Inorganic materials 0.000 description 2
- 230000035515 penetration Effects 0.000 description 2
- 230000008569 process Effects 0.000 description 2
- 239000000377 silicon dioxide Substances 0.000 description 2
- 238000007581 slurry coating method Methods 0.000 description 2
- 239000000243 solution Substances 0.000 description 2
- 229910001018 Cast iron Inorganic materials 0.000 description 1
- YKTSYUJCYHOUJP-UHFFFAOYSA-N [O--].[Al+3].[Al+3].[O-][Si]([O-])([O-])[O-] Chemical group [O--].[Al+3].[Al+3].[O-][Si]([O-])([O-])[O-] YKTSYUJCYHOUJP-UHFFFAOYSA-N 0.000 description 1
- 238000007605 air drying Methods 0.000 description 1
- 229910045601 alloy Inorganic materials 0.000 description 1
- 239000000956 alloy Substances 0.000 description 1
- 239000002518 antifoaming agent Substances 0.000 description 1
- 230000015572 biosynthetic process Effects 0.000 description 1
- 239000000919 ceramic Substances 0.000 description 1
- 230000000052 comparative effect Effects 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 230000008030 elimination Effects 0.000 description 1
- 238000003379 elimination reaction Methods 0.000 description 1
- 230000002708 enhancing effect Effects 0.000 description 1
- 238000010304 firing Methods 0.000 description 1
- 239000012530 fluid Substances 0.000 description 1
- 230000008595 infiltration Effects 0.000 description 1
- 238000001764 infiltration Methods 0.000 description 1
- 238000002347 injection Methods 0.000 description 1
- 239000007924 injection Substances 0.000 description 1
- 238000001746 injection moulding Methods 0.000 description 1
- 238000003780 insertion Methods 0.000 description 1
- 230000037431 insertion Effects 0.000 description 1
- 238000005495 investment casting Methods 0.000 description 1
- 239000002245 particle Substances 0.000 description 1
- 230000003252 repetitive effect Effects 0.000 description 1
- 230000004044 response Effects 0.000 description 1
- 239000007787 solid Substances 0.000 description 1
- 230000007847 structural defect Effects 0.000 description 1
- 239000000725 suspension Substances 0.000 description 1
- 230000008646 thermal stress Effects 0.000 description 1
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 1
- 239000000080 wetting agent Substances 0.000 description 1
Images
Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
- B22D18/00—Pressure casting; Vacuum casting
- B22D18/06—Vacuum casting, i.e. making use of vacuum to fill the mould
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22C—FOUNDRY MOULDING
- B22C9/00—Moulds or cores; Moulding processes
- B22C9/02—Sand moulds or like moulds for shaped castings
- B22C9/04—Use of lost patterns
Definitions
- the present invention relates to a method of casting molten metal according to the preamble of claim 1 and to an apparatus for countergravity casting of molten metal, respectively, according to the preamble of claim 12.
- the present invention relates to the countergravity casting of molten metal using a gas permeable investment shell mold provided with a thin mold wall that better tolerates pattern removal stresses and that is supported in a compacted particulate support media during the casting process.
- a plurality of expendable (e.g., meltable) patterns of the article to be cast are first formed and then assembled with suitable ingate patterns and the like to form a pattern assembly or tree.
- the pattern assembly is then invested with refractory particulate by alternately dipping the pattern assembly in a refractory slurry (comprising refractory powder and a suitable binder solution capable of hardening during drying under ambient conditions) and then dusted or stuccoed with coarser refractory powder.
- the sequence of dipping and stuccoing is repeated to build up a multi-layered refractory shell having a sufficient thickness to resist stresses imparted thereto by subsequent pattern removal, firing and metal casting operations.
- the pattern removal operation typically has been carried out by steam autoclaving wherein the invested pattern assembly is placed in a steam autoclave heated to a temperature in the range of about 135°C (275°F) to about 177°C (350°F) to melt out the pattern from the refractory shell.
- prior art workers have experienced damage (e.g., cracking) to the refractory shell during the steam autoclaving step as a result of thermal expansion of the pattern (e.g., wax) relative to the refractory shell.
- damage e.g., cracking
- prior art workers have increased the thickness of the shell to better withstand these stresses.
- Aforementioned US-A-4,791,977 describes stresses imposed on the refractory shell mold during the vacuum-assisted countergravity casting of molten metal therein.
- that patent recognizes that harmful stresses can be imposed on the shell as a result of internal metallostatic pressure exerted thereon by the metal cast therein in conjunction with the external vacuum applied about the shell mold during the casting process.
- the patent recognizes that such stresses when combined with the high temperatures of the metal in the shell mold can cause shell wall movement, metal penetration into the walls, metal leakage and outright failure of the shell mold, especially if there are any structural defects in the shell.
- this patent provides a means of reducing such stresses on the investment shell mold (i.e., by using a differential pressure technique between the internal mold fill passage and vacuum chamber external of the shell), the investment shell mold used in that patent is still required to have a conventional shell wall thickness and strength to withstand stresses during pattern removal and molten metal casting.
- the present invention contemplates an improved, economical countergravity casting method and apparatus involving forming an expendable pattern of an article to be cast and comprising a meltable material that expands upon heating, investing the pattern with particulate mold material in multiple layers so controlled as to form a refractory shell having a wall thickness not exceeding about 3 mm (.12 inch) about the pattern and heating the invested pattern, such as by steam autoclaving, to remove the pattern from the shell to leave a mold cavity therein.
- a refractory particulate support media is disposed about the thin shell mold with the mold cavity communicated to a lower molten metal inlet disposed external of the support media.
- the mold cavity is evacuated while, concurrently, a pressure is so applied to the support media as to compress the support media about the refractory shell to support the shell against casting stresses when the molten metal is countergravity cast into the evacuated mold cavity while the molten metal inlet is communicated to a source of molten metal.
- a refractory shell mold having a wall thickness not exceeding about 3 mm (.12 inch) in the practice of the invention is based on the discovery, contrary to accepted prior art practice, that such thin shell walls are better able to tolerate stresses imposed thereon by pattern expansion during the pattern removal operation.
- the invention relates to the discovery that the permeability of thin shell molds does not increase in direct proportion to reductions in shell wall thickness but rather in an unexpectedly greater manner.
- such thin shell walls i.e., not exceeding about 3 mm (.12 inch) wall thickness
- This increased shell permeability has been found to relieve the stresses imposed on the shell during pattern removal by steam autoclaving by enhancing infiltration into the shell of the molten skin initially melted on the pattern. Moreover, the increased shell mold permeability shortens the time for pattern removal by facilitating ingress of the steam to the pattern surface.
- a refractory shell mold having such thin wall thickness i.e., not exceeding about 3 mm (.12 inch)
- Use of a refractory shell mold having such thin wall thickness is also based on the discovery that such a thin shell mold can be adequately supported to withstand stresses imposed thereon during differential pressure, countergravity casting by rigidizing or compressing a particulate support media about the shell concurrent with the mold cavity being evacuated.
- the thin shell mold is disposed in a loose particulate support media (e.g., loose foundry sand) contained in a vacuum housing and a pressure transmitting means is moved relative to the vacuum housing and the support media to compress the support media about the shell when the vacuum housing is evacuated to evacuate the mold cavity for casting.
- the pressure transmitting means may comprise a movable wall of the vacuum housing that is subjected to ambient pressure on the outside and a relative vacuum on the inside to so compress the support media about the shell mold as to support the shell against casting stresses.
- the pressure transmitting means may alternately comprise a pressurized bladder disposed in contact with the support media for compressing the support media about the thin shell for the same purpose.
- Figure 1 is a side elevation of a pattern assembly.
- Figure 2 is a sectioned, side elevation of the pattern assembly after investing with particulate mold material.
- Figure 3 is a sectioned, side elevation of the thin shell mold after the pattern assembly is removed by steam autoclaving
- Figure 4 is a sectioned side elevation of a countergravity casting apparatus in accordance with the invention wherein the shell mold is disposed in rigidized particulate support media in a vacuum housing and the external molten metal inlet to the shell mold is immersed in an underlying pool of molten metal.
- Figure 5 is a sectioned side elevation of a countergravity casting apparatus in accordance with another embodiment of the invention.
- Figure 6 is a sectioned side elevation of a countergravity casting apparatus in accordance with still another embodiment of the invention.
- Fig. 1 an expendable pattern assembly or tree 10 comprising a central, cylindrical riser-forming portion 12 and a plurality of mold cavity-forming portions 14 each connected to the riser-forming portion 12 by a respective ingate-forming portion 16.
- the mold cavity-forming portions 14 are configured in the shape of the article or part to be cast and are spaced apart about the periphery of the riser-forming portion 12 and along the length thereof as shown.
- each mold cavity-forming portion 14 and its respective ingate-forming portion 16 are injection molded and then manually attached (e.g., wax welded or adhered) onto the riser-forming portion 12.
- the riser-forming portion 12 is formed by injection molding as a separate piece.
- a refractory frusto-conical collar 18 is attached (e.g., wax welded or adhered) to the lower end of the riser-forming portion 12.
- the pattern assembly 10 is preferably made of a meltable, solid (non-porous) material which expands upon heating as will be explained.
- Wax is the preferred material for the pattern assembly due to its low cost and predictable properties.
- the pattern wax melts in the range of about 54.4°C (130°F) to about 65.6°C (150°F).
- wax viscosity must be selected to avoid shell cracking during the pattern removal (e.g., wax viscosity at 76.7°C (170°F) should be less than 130 cPs (1300 centipoise)).
- Urea may also be useful as a pattern material and melts in the range of about 112.8°C (235°F) to about 129.4°C (265°F).
- the various portions 12,14,16 of the pattern assembly 10 be made of the same pattern material so long as the pattern assembly 10 is subsequently removable by heating, such as steam autoclaving, as will be described.
- the pattern assembly 10 is invested with multiple layers of refractory material 22 to form a thin shell 30 thereabout.
- the pattern assembly is invested by repeatedly dipping it in a refractory slurry (not shown) comprising a suspension of a refractory powder (e.g., zircon, alumina, fused silica and others) in a binder solution, such as ethyl silicate or colloidal silica sol, and small amounts of an organic film former, a wetting agent and a defoaming agent.
- a refractory slurry comprising a suspension of a refractory powder (e.g., zircon, alumina, fused silica and others) in a binder solution, such as ethyl silicate or colloidal silica sol, and small amounts of an organic film former, a wetting agent and a defoaming agent.
- a binder solution such as ethyl silicate or colloidal silica sol
- the slurry coating is hardened using forced air drying or other means to form a refractory layer on the pattern assembly 10 or on the previously formed refractory layer.
- This sequence of dipping, stuccoing and drying is repeated until a multi-layered shell 30 of desired wall thickness t about the mold cavity-forming portions 14 is built up.
- the shell formation process i.e., dipping, stuccoing and drying
- a multi-layer refractory shell 30 having a maximum wall thickness t not exceeding about 3 mm (.12 inch) about the mold cavity-forming portions 14.
- this wall thickness has been discovered to exhibit a surprising ability to accommodate stresses imposed on the shell during pattern removal by steam autoclaving.
- a shell wall thickness not exceeding about 3 mm (.12 inch) is built up or comprised of four to five refractory layers formed by the repetitive dipping, stuccoing and drying sequence described hereinabove.
- Fig. 3 illustrates the refractory shell 30 after removal of the pattern assembly 10 by steam autoclaving.
- the refractory shell 30 is shown positioned inside a steam autoclave 34 (schematically shown) of conventional type; e.g., model 286PT available from Leeds and Bradford Co.
- a steam autoclave 34 (schematically shown) of conventional type; e.g., model 286PT available from Leeds and Bradford Co.
- removal of the pattern assembly 10 leaves a thin refractory shell 30 having the mold cavities 36 interconnected to the central riser 37 via the respective lateral ingates 38.
- the riser 37 is open at the lower and upper ends.
- the invested pattern assembly 40 Fig. 2 is subjected to steam at a temperature of about 135°C (275°F) to about 177°C (350°F) (steam pressure of about 5.62 bar (80 psi) to about 7.73 bar (110 psi)) for a time sufficient to melt the pattern assembly 10 out of the refractory shell 30.
- a molten surface film is melted on the pattern assembly 10 by ingress of the steam through the gas permeable refractory shell 30.
- the permeability of the thin refractory shell 30 is surprisingly and unexpectedly able to absorb a major portion of this initial melted surface film and thereby relieve pattern expansion forces that would otherwise be exerted on the shell 30.
- the remainder of the pattern assembly 10 is melted and, for the most part, drains from the refractory shell 30 through the opening 18a in the collar 18 therein.
- the wall thickness of the refractory shell 30 is controlled in accordance with the invention so as not to exceed about 3 mm (.12 inch).
- This shell wall thickness has been found to exhibit an unexpectedly high permeability (e.g., as measured by a known nitrogen permeability test conducted at 1038°C (1900°F) adopted by the Investment Casting Institute) for absorbing the initial melted surface film of pattern material during steam autoclaving.
- a refractory shell 30 (fired) at about 982°C (1800°F) having a wall thickness of about 3 mm (.12 inch) (4 refractory layers) has been measured (by the aforementioned nitrogen permeability test) to exhibit a gas permeability of more than twice that exhibited by a like shell having twice the thickness (i.e., a shell wall thickness of 6 mm (.25 inch) and comprising eight refractory layers).
- the gas permeability of the fired refractory shell 30 of 3 mm (.12 inch) wall thickness was measured as 316-468 cc of N2/minute compared to only 80-120 cc of N2/minute for the like shell of 6 mm (.25 inch) wall thickness.
- the fired refractory shell 30 is so formed in accordance with the invention as to exhibit a gas permeability of at least generally three times that of a like shell of twice the wall thickness.
- this unexpectedly high permeability of the thin refractory shell 30 enhances the ability of the refractory shell wall to absorb the initial melted surface film on the pattern assembly 10 formed during steam autoclaving to relieve any stresses that would normally be imposed on the shell as a result of thermal expansion of the pattern assembly 10 relative to the refractory shell 30.
- a decreased (thinner) shell wall thickness provides a significantly improved response to steam autoclaving with reduced shell distortion and damage, such as cracking. Not only is shell distortion and damage reduced but also the time required for pattern removal by steam autoclaving is substantially reduced by virtue of better ingress of the steam through the high permeability shell 30 and resulting faster heating of the pattern assembly 10.
- the quantity of refractory particulate required for the refractory shell 30 is significantly reduced since a thinner shell wall thickness is used.
- the cost of casting is thereby significantly reduced; e.g., cost reductions of 40% to 75% are achievable based upon savings in the amount of refractory material used.
- the use of a thin-walled shell mold permits closer spacing of the mold cavity-forming portions 14 and the ingates 16 to substantially increase the number of castings that can be made per mold. Overall production output is increased at reduced cost in like manner (except for wall thickness).
- Table I sets forth comparative data relating to a so-called loading factor (i.e., the number of parts castable per mold) for a given part (e.g., an automobile rocker, window latch and a cleat) when using thick-walled shells (i.e., 6 mm (.25 inch) shell wall thickness) and when using the thin-walled shells 30 of the invention.
- a so-called loading factor i.e., the number of parts castable per mold
- a given part e.g., an automobile rocker, window latch and a cleat
- thick-walled shells i.e., 6 mm (.25 inch) shell wall thickness
- Both the thick-walled shell (9 slurry dips/stuccoes) and the thin-walled shell (4-5 slurry dips/stuccoes) were prepared in like manner using like slurries and stuccoes (e.g., initial slurry dip containing 200 mesh fused silica (15.2 weight %) and 325 mesh zircon (56.9 weight %), colloidal silica sol binder (17.8 weight %) and water (10.1 weight %) and subsequent slurry dips containing Mulgrain® M-47 mullite (15.1 weight %), 200 mesh fused silica (25.2 weight %) and 600 mesh zircon (35.3 weight %), ethyl silicate binder (15.6 weight %) and isopropanol (8.8 weight %) and stuccoed in sequence by about 100 mesh zircon, 60 mesh Mulgrain M-47 mullite and the balance being stuccoed by about 25 mesh Mulgrain M-47 mullite.
- the shells were
- the thinner shell molds of the invention significantly increase the loading factor (i.e., parts castable per mold) and significantly reduce the amount of refractory material needed to form the fired shell. All this is achieved while also achieving equivalent or better values for mold distortion and damage during the steam autoclaving operation.
- molten metal is differential pressure, countergravity cast into the thin shell mold 30 (after the shell is fired at about 982°C (1800°F)) as illustrated in Fig. 4.
- the thin shell mold 30 is supported in a loose refractory particulate support media 60 itself contained in a vacuum housing 70.
- the vacuum housing 70 includes a bottom support wall 72, an upstanding side wall 73 and a movable top end wall 74 defining therewithin a vacuum chamber 76.
- the bottom wall 72 and the upstanding side wall 73 are formed of gas impermeable material, such as metal, while the movable top end wall 74 comprises a gas permeable (porous) plate 75 having a vacuum plenum 77 connected thereto to define a vacuum chamber 78 above (outside) the gas permeable plate 75.
- the vacuum chamber 78 is connected to a source of vacuum, such as a vacuum pump 80, by conduit 82.
- the movable top end wall 74 includes a peripheral seal 84 that sealingly engages the interior of the upstanding side wall 73 to allow movement of the top end wall 74 relative to the side wall 73 while maintaining a vacuum seal therebetween.
- a ceramic fill tube 90 disposed in housing 70 and providing a lower molten metal inlet to the mold cavities 36 via the riser 37 and the respective ingates 38 is sealingly engaged to the frusto-conical collar 18 as the mold is placed thereon.
- a refractory cap 20 is placed atop the shell mold to close off the upper end of the riser 37.
- the loose refractory particulate support media 60 e.g., loose foundry silica sand of about 60 mesh
- the housing 70 is vibrated to aid in settling of the support media 60 in the chamber 76 about the shell 30.
- the movable top end wall 74 is then positioned in the open upper end of the housing 70 with the peripheral seal 84 sealingly engaging the upstanding side wall 73 and with the inner side of the gas permeable plate 75 facing and in contact with the support media 60, Fig. 4.
- the casting apparatus 100 is positioned above a source 102 (e.g., a pool) of the molten metal 104 to be cast.
- a source 102 e.g., a pool
- the molten metal 104 is contained in a casting vessel 106.
- a vacuum is then drawn in the vacuum chamber 78 of the vacuum bell 77 and hence in the vacuum chamber 76 through the gas permeable plate 75 by actuation of the vacuum pump 80.
- Evacuation of the chamber 76 evacuates the mold cavities 36 through the thin gas permeable shell wall.
- the level of vacuum in chamber 76 is selected sufficient to draw the molten metal 104 upwardly from the pool 102 into the mold cavities 36 when the fill tube 90 is immersed in the molten metal 104 as shown in Fig. 4.
- the top end wall 74 When the vacuum is drawn in vacuum chambers 76,78, the top end wall 74 is subjected to atmospheric (or ambient) pressure on the side thereof external of the peripheral seal 84 while the inner side of the plate 75 is subjected to a relative vacuum.
- This pressure differential across the top end wall 74 causes the top wall 74 to move downwardly relatively to side wall 73 and causes the plate 75 to exert sufficient pressure on the support media 60 so as to compress or rigidize the support media 60 about the shell 30 to support it against casting stresses.
- a pressure is applied concurrently by the plate 75 to compress the support media 60 about the shell 30 to support it against casting stresses.
- the amount of pressure applied by the plate 75 to compress the support media 60 can be controlled by controlling the level of vacuum established in the vacuum chamber 76.
- the molten metal 104 will be drawn upwardly through the fill tube 90 through the riser 37 and into the mold cavities 36 via the lateral ingates 38. The molten metal 104 is thereby vacuum countergravity cast into the mold cavities 36.
- This reduction in stress in conjunction with support of the shell 30 by the support media 60 permits countergravity casting of the high temperature molten metal 104 into the thin shell 30 (having a wall thickness not exceeding about .12 inch) without harmful mold wall movement and molten metal penetration into the mold wall.
- the surrounding support media 60 also aids in preventing the molten metal 104 from leaking through the defect and, in any event, confines any leakage in proximity to the shell 30 to prevent damage to the casting apparatus, permitting vacuum to be held until the castings solidify.
- the casting assembly 100 is moved upwardly to remove the fill tube 90 from the molten metal pool.
- the top wall 74 of the housing 70 is then removed at an unload station (not shown) to allow removal of the support media 60 and the metal-filled shell 30 from the vacuum chamber 76.
- the support media 60 can be recycled for reuse in casting another shell 30.
- the metal-filled shell 30 is allowed to cool to ambient.
- the shell 30 is easily removed from the solidified casting by virtue of its thin wall thickness. For example, cooling of the metal-filled shell 30 often causes the shell 30 to simply pop off the casting due to thermal stresses imposed on the shell during cooling. In general, considerably less time is required to remove the thin shell 30 than to remove shell molds having thicker wall thicknesses heretofore used.
- a casting apparatus 100' of another embodiment of the invention is illustrated.
- like features of Fig. 4 are represented by like reference numerals primed.
- the casting apparatus 100' of Fig. 5 differs from the casting apparatus 100 of Fig. 4 in using an annular vacuum bell 110' about the housing 70' and a flexible, gas impermeable membrane 112' sealingly disposed on the open upper end of the housing 70' (providing a movable housing top end wall) for applying a pressure to the support media 60' when the housing 70' is evacuated.
- the vacuum plenum 110' defines an annular vacuum chamber 114' about the vacuum chamber 76' of the housing 70' and is interconnected thereto by an annular gas permeable (porous) side wall housing section 116'.
- the flexible, gas impermeable membrane 112' When the vacuum is drawn in the vacuum chamber 76', the flexible, gas impermeable membrane 112' is subjected to atmospheric pressure on the outside surface 112a' and to the relative vacuum on the inside surface 112b', causing the membrane 112' to compress the loose refractory particulate support media 60' about the thin shell mold 30' to support it against casting stresses in the manner described hereinabove with respect to Fig. 4 as the molten metal is urged upwardly from the underlying pool through the fill tube 90' and the riser 37' and into the mold cavities 36' via the ingates 38'.
- the embodiment of Fig. 5 functions and offers advantages described above for the embodiment of Fig. 4.
- FIG. 6 a casting apparatus 100'' of still another embodiment of the invention is illustrated wherein like features of Fig. 4 are represented by like reference numerals double primed.
- the embodiment of Fig. 6 differs from that of Fig. 4 in using one or more annular fluid pressurizable bladders 120'' (one shown) disposed in contact with the refractory particulate support media 60" in the housing 70'' to exert a pressure on the support media 60'' to compress it about the thin shell mold 30'' when the mold cavities 36'' are evacuated during countergravity casting.
- the housing 70'' includes a non-movable top end wall 74'' which comprises a gas permeable plate 75'' sealed to the top of the housing 70'' by seal 84'' and a vacuum bell 77'' connected to the plate 75''.
- the vacuum chamber 78'' of the bell 77'' overlies the gas permeable portion 75a'' of the plate 75'' so as to evacuate the vacuum chamber 76'' of the housing 70'' by a means of vacuum pump 80'' communicating thereto via conduit 82''.
- the bladder 120'' is pressurized by a suitable gas supply 121'', such as compressed air, through suitable gas supply pipes 122''. Pressurization of the bladder 120'' exerts a pressure on the refractory particulate support media 60'' to compress it about the shell 30'' to support it against casting stresses in the same manner as described hereinabove for the preceding embodiments.
- a suitable gas supply 121'' such as compressed air
- Pressurization of the bladder 120'' exerts a pressure on the refractory particulate support media 60'' to compress it about the shell 30'' to support it against casting stresses in the same manner as described hereinabove for the preceding embodiments.
- the embodiment of Fig. 6 is similar in function to the preceding embodiments of Figs. 4 and 5.
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Description
- The present invention relates to a method of casting molten metal according to the preamble of
claim 1 and to an apparatus for countergravity casting of molten metal, respectively, according to the preamble ofclaim 12. - Such method and apparatus, respectively, are known from EP-B-0 019 971. This document discloses the insertion of a thin shell of multiple layers after the elimination of a model or expendable pattern in a particulate refractory supporting media.
- More specifically, the present invention relates to the countergravity casting of molten metal using a gas permeable investment shell mold provided with a thin mold wall that better tolerates pattern removal stresses and that is supported in a compacted particulate support media during the casting process.
- Generally, vacuum-assisted countergravity casting processes using gas permeable investment shell molds are described in US-A-3,900,064; US-A-4,340,108; US-A-4,532,976; US-A-4,589,466 and US-A-4,791,977.
- In the fabrication of the gas permeable, high temperature bonded refractory investment shell molds for use in such countergravity casting processes, a plurality of expendable (e.g., meltable) patterns of the article to be cast are first formed and then assembled with suitable ingate patterns and the like to form a pattern assembly or tree. The pattern assembly is then invested with refractory particulate by alternately dipping the pattern assembly in a refractory slurry (comprising refractory powder and a suitable binder solution capable of hardening during drying under ambient conditions) and then dusted or stuccoed with coarser refractory powder. The sequence of dipping and stuccoing is repeated to build up a multi-layered refractory shell having a sufficient thickness to resist stresses imparted thereto by subsequent pattern removal, firing and metal casting operations.
- In particular, the pattern removal operation typically has been carried out by steam autoclaving wherein the invested pattern assembly is placed in a steam autoclave heated to a temperature in the range of about 135°C (275°F) to about 177°C (350°F) to melt out the pattern from the refractory shell. In the past, prior art workers have experienced damage (e.g., cracking) to the refractory shell during the steam autoclaving step as a result of thermal expansion of the pattern (e.g., wax) relative to the refractory shell. In efforts to reduce or minimize damage (e.g., cracking) of the refractory shell during the steam autoclaving step, prior art workers have increased the thickness of the shell to better withstand these stresses. Unfortunately, increasing the thickness of the refractory shell results in heavier investment shell molds and consumption of greater quantities of refractory material, adding to the cost of casting. Moreover, the greater thickness of the refractory shell also requires conduct of the steam autoclaving operation for longer times to effect removal of the pattern from the invested pattern assembly. Typically, investment shell molds used to countergravity cast iron base and other alloys using the aforementioned patented processes are made to have a shell wall thickness of at least about 0.64 mm (1/4 inch) to this end.
- Aforementioned US-A-4,791,977 describes stresses imposed on the refractory shell mold during the vacuum-assisted countergravity casting of molten metal therein. In particular, that patent recognizes that harmful stresses can be imposed on the shell as a result of internal metallostatic pressure exerted thereon by the metal cast therein in conjunction with the external vacuum applied about the shell mold during the casting process. The patent recognizes that such stresses when combined with the high temperatures of the metal in the shell mold can cause shell wall movement, metal penetration into the walls, metal leakage and outright failure of the shell mold, especially if there are any structural defects in the shell. Although this patent provides a means of reducing such stresses on the investment shell mold (i.e., by using a differential pressure technique between the internal mold fill passage and vacuum chamber external of the shell), the investment shell mold used in that patent is still required to have a conventional shell wall thickness and strength to withstand stresses during pattern removal and molten metal casting.
- It is an object of the present invention to provide an improved, economical countergravity casting method and apparatus that uses a refractory investment shell mold having a significantly reduced wall thickness that nevertheless is less subject to damage (e.g., cracking) during operations such as pattern removal by steam autoclaving.
- This object according to the present invention is accomplished by means of the method according to
claim 1 and, as far as the apparatus is concerned, by means of the apparatus according toclaim 12. - It is an advantage of the present invention that it provides an improved, economical countergravity casting method and apparatus which significantly reduces the amount of bonded refractory material needed to fabricate the investment shell mold.
- It is another advantage of the invention that it provides an improved, economical countergravity casting method and apparatus which significantly increases the number of castings that can be cast per investment shell mold.
- It is still a further advantage of the invention that it provides an improved countergravity casting method and apparatus which reduces stresses imposed on the investment shell mold by the presence of internal metallostatic pressure and external vacuum conditions about the shell during casting.
- It is still a further advantage of the invention that it provides an improved countergravity casting method and apparatus which supports the investment shell mold during casting in such a manner as to prevent damage to the mold from casting stresses, permit larger molds to be cast and to prevent molten metal leakage therefrom.
- Thus, the present invention contemplates an improved, economical countergravity casting method and apparatus involving forming an expendable pattern of an article to be cast and comprising a meltable material that expands upon heating, investing the pattern with particulate mold material in multiple layers so controlled as to form a refractory shell having a wall thickness not exceeding about 3 mm (.12 inch) about the pattern and heating the invested pattern, such as by steam autoclaving, to remove the pattern from the shell to leave a mold cavity therein. After the thin shell is fired to impart desired mold strength thereto, a refractory particulate support media is disposed about the thin shell mold with the mold cavity communicated to a lower molten metal inlet disposed external of the support media.
- After the thin refractory shell mold is surrounded by the particulate support media, the mold cavity is evacuated while, concurrently, a pressure is so applied to the support media as to compress the support media about the refractory shell to support the shell against casting stresses when the molten metal is countergravity cast into the evacuated mold cavity while the molten metal inlet is communicated to a source of molten metal.
- Use of a refractory shell mold having a wall thickness not exceeding about 3 mm (.12 inch) in the practice of the invention is based on the discovery, contrary to accepted prior art practice, that such thin shell walls are better able to tolerate stresses imposed thereon by pattern expansion during the pattern removal operation. In particular, the invention relates to the discovery that the permeability of thin shell molds does not increase in direct proportion to reductions in shell wall thickness but rather in an unexpectedly greater manner. For example, such thin shell walls (i.e., not exceeding about 3 mm (.12 inch) wall thickness) have been found to exhibit a gas permeability of more than twice, generally greater than three times, the gas permeability of a like shell mold having twice the wall thickness.
- This increased shell permeability has been found to relieve the stresses imposed on the shell during pattern removal by steam autoclaving by enhancing infiltration into the shell of the molten skin initially melted on the pattern. Moreover, the increased shell mold permeability shortens the time for pattern removal by facilitating ingress of the steam to the pattern surface.
- Use of a refractory shell mold having such thin wall thickness (i.e., not exceeding about 3 mm (.12 inch)) in practicing the invention is also based on the discovery that such a thin shell mold can be adequately supported to withstand stresses imposed thereon during differential pressure, countergravity casting by rigidizing or compressing a particulate support media about the shell concurrent with the mold cavity being evacuated.
- For example, in one embodiment of the invention, the thin shell mold is disposed in a loose particulate support media (e.g., loose foundry sand) contained in a vacuum housing and a pressure transmitting means is moved relative to the vacuum housing and the support media to compress the support media about the shell when the vacuum housing is evacuated to evacuate the mold cavity for casting. The pressure transmitting means may comprise a movable wall of the vacuum housing that is subjected to ambient pressure on the outside and a relative vacuum on the inside to so compress the support media about the shell mold as to support the shell against casting stresses. The pressure transmitting means may alternately comprise a pressurized bladder disposed in contact with the support media for compressing the support media about the thin shell for the same purpose.
- The aforementioned objects and advantages of the present invention will become more readily apparent from the following detailed description and drawings.
- Figure 1 is a side elevation of a pattern assembly.
- Figure 2 is a sectioned, side elevation of the pattern assembly after investing with particulate mold material.
- Figure 3 is a sectioned, side elevation of the thin shell mold after the pattern assembly is removed by steam autoclaving
- Figure 4 is a sectioned side elevation of a countergravity casting apparatus in accordance with the invention wherein the shell mold is disposed in rigidized particulate support media in a vacuum housing and the external molten metal inlet to the shell mold is immersed in an underlying pool of molten metal.
- Figure 5 is a sectioned side elevation of a countergravity casting apparatus in accordance with another embodiment of the invention.
- Figure 6 is a sectioned side elevation of a countergravity casting apparatus in accordance with still another embodiment of the invention.
- Referring now to the drawings, there is shown in Fig. 1 an expendable pattern assembly or
tree 10 comprising a central, cylindrical riser-formingportion 12 and a plurality of mold cavity-formingportions 14 each connected to the riser-formingportion 12 by a respective ingate-formingportion 16. The mold cavity-formingportions 14 are configured in the shape of the article or part to be cast and are spaced apart about the periphery of the riser-formingportion 12 and along the length thereof as shown. Typically, each mold cavity-formingportion 14 and its respective ingate-formingportion 16 are injection molded and then manually attached (e.g., wax welded or adhered) onto the riser-formingportion 12. The riser-formingportion 12 is formed by injection molding as a separate piece. - A refractory frusto-
conical collar 18 is attached (e.g., wax welded or adhered) to the lower end of the riser-formingportion 12. - The
pattern assembly 10 is preferably made of a meltable, solid (non-porous) material which expands upon heating as will be explained. Wax is the preferred material for the pattern assembly due to its low cost and predictable properties. In general, the pattern wax melts in the range of about 54.4°C (130°F) to about 65.6°C (150°F). Importantly, wax viscosity must be selected to avoid shell cracking during the pattern removal (e.g., wax viscosity at 76.7°C (170°F) should be less than 130 cPs (1300 centipoise)). Urea may also be useful as a pattern material and melts in the range of about 112.8°C (235°F) to about 129.4°C (265°F). - It is not necessary in practicing the present invention that the
various portions pattern assembly 10 be made of the same pattern material so long as thepattern assembly 10 is subsequently removable by heating, such as steam autoclaving, as will be described. - Referring now to Fig. 2, the
pattern assembly 10 is invested with multiple layers ofrefractory material 22 to form athin shell 30 thereabout. The pattern assembly is invested by repeatedly dipping it in a refractory slurry (not shown) comprising a suspension of a refractory powder (e.g., zircon, alumina, fused silica and others) in a binder solution, such as ethyl silicate or colloidal silica sol, and small amounts of an organic film former, a wetting agent and a defoaming agent. After each dipping, excess slurry is allowed to drain off and the slurry coating on the pattern assembly is stuccoed or sanded with dry refractory particles. Suitable refractory materials for stuccoing include granular zircon, fused silica, silica, various aluminum silicate groups including mullite, fused alumina and similar materials. - After each sequence of dipping and stuccoing, the slurry coating is hardened using forced air drying or other means to form a refractory layer on the
pattern assembly 10 or on the previously formed refractory layer. This sequence of dipping, stuccoing and drying is repeated until amulti-layered shell 30 of desired wall thickness t about the mold cavity-formingportions 14 is built up. - In accordance with the present invention, the shell formation process (i.e., dipping, stuccoing and drying) is controlled to form a multi-layer
refractory shell 30 having a maximum wall thickness t not exceeding about 3 mm (.12 inch) about the mold cavity-formingportions 14. As will be explained hereinbelow, this wall thickness has been discovered to exhibit a surprising ability to accommodate stresses imposed on the shell during pattern removal by steam autoclaving. In general, a shell wall thickness not exceeding about 3 mm (.12 inch) is built up or comprised of four to five refractory layers formed by the repetitive dipping, stuccoing and drying sequence described hereinabove. - Fig. 3 illustrates the
refractory shell 30 after removal of thepattern assembly 10 by steam autoclaving. In particular, therefractory shell 30 is shown positioned inside a steam autoclave 34 (schematically shown) of conventional type; e.g., model 286PT available from Leeds and Bradford Co. As is apparent, removal of thepattern assembly 10 leaves a thinrefractory shell 30 having themold cavities 36 interconnected to thecentral riser 37 via therespective lateral ingates 38. At this stage of processing, theriser 37 is open at the lower and upper ends. - During the steam autoclaving operation, the invested
pattern assembly 40, Fig. 2, is subjected to steam at a temperature of about 135°C (275°F) to about 177°C (350°F) (steam pressure of about 5.62 bar (80 psi) to about 7.73 bar (110 psi)) for a time sufficient to melt thepattern assembly 10 out of therefractory shell 30. - In particular, during the initial stages of steam autoclaving, a molten surface film is melted on the
pattern assembly 10 by ingress of the steam through the gas permeablerefractory shell 30. As will be explained hereinbelow, the permeability of the thinrefractory shell 30 is surprisingly and unexpectedly able to absorb a major portion of this initial melted surface film and thereby relieve pattern expansion forces that would otherwise be exerted on theshell 30. Over time, the remainder of thepattern assembly 10 is melted and, for the most part, drains from therefractory shell 30 through the opening 18a in thecollar 18 therein. - As mentioned hereinabove, the wall thickness of the
refractory shell 30 is controlled in accordance with the invention so as not to exceed about 3 mm (.12 inch). This shell wall thickness has been found to exhibit an unexpectedly high permeability (e.g., as measured by a known nitrogen permeability test conducted at 1038°C (1900°F) adopted by the Investment Casting Institute) for absorbing the initial melted surface film of pattern material during steam autoclaving. For example, a refractory shell 30 (fired) at about 982°C (1800°F) having a wall thickness of about 3 mm (.12 inch) (4 refractory layers) has been measured (by the aforementioned nitrogen permeability test) to exhibit a gas permeability of more than twice that exhibited by a like shell having twice the thickness (i.e., a shell wall thickness of 6 mm (.25 inch) and comprising eight refractory layers). In particular, the gas permeability of the firedrefractory shell 30 of 3 mm (.12 inch) wall thickness was measured as 316-468 cc of N₂/minute compared to only 80-120 cc of N₂/minute for the like shell of 6 mm (.25 inch) wall thickness. - Preferably, the fired
refractory shell 30 is so formed in accordance with the invention as to exhibit a gas permeability of at least generally three times that of a like shell of twice the wall thickness. - As already mentioned, this unexpectedly high permeability of the thin refractory shell 30 (not exceeding 3 mm (.12 inch) wall thickness) enhances the ability of the refractory shell wall to absorb the initial melted surface film on the
pattern assembly 10 formed during steam autoclaving to relieve any stresses that would normally be imposed on the shell as a result of thermal expansion of thepattern assembly 10 relative to therefractory shell 30. Contrary to the prior art practice of increasing the shell wall thickness to withstand such stresses during pattern removal, the present invention has discovered that a decreased (thinner) shell wall thickness provides a significantly improved response to steam autoclaving with reduced shell distortion and damage, such as cracking. Not only is shell distortion and damage reduced but also the time required for pattern removal by steam autoclaving is substantially reduced by virtue of better ingress of the steam through thehigh permeability shell 30 and resulting faster heating of thepattern assembly 10. - Moreover, as will become apparent from the examples set forth in Table I below, the quantity of refractory particulate required for the
refractory shell 30 is significantly reduced since a thinner shell wall thickness is used. The cost of casting is thereby significantly reduced; e.g., cost reductions of 40% to 75% are achievable based upon savings in the amount of refractory material used. - In addition, the use of a thin-walled shell mold permits closer spacing of the mold cavity-forming
portions 14 and theingates 16 to substantially increase the number of castings that can be made per mold. Overall production output is increased at reduced cost in like manner (except for wall thickness). - After steam autoclaving the shell is fired at about 1800°F for 90 minutes.
- Table I sets forth comparative data relating to a so-called loading factor (i.e., the number of parts castable per mold) for a given part (e.g., an automobile rocker, window latch and a cleat) when using thick-walled shells (i.e., 6 mm (.25 inch) shell wall thickness) and when using the thin-
walled shells 30 of the invention. Both the thick-walled shell (9 slurry dips/stuccoes) and the thin-walled shell (4-5 slurry dips/stuccoes) were prepared in like manner using like slurries and stuccoes (e.g., initial slurry dip containing 200 mesh fused silica (15.2 weight %) and 325 mesh zircon (56.9 weight %), colloidal silica sol binder (17.8 weight %) and water (10.1 weight %) and subsequent slurry dips containing Mulgrain® M-47 mullite (15.1 weight %), 200 mesh fused silica (25.2 weight %) and 600 mesh zircon (35.3 weight %), ethyl silicate binder (15.6 weight %) and isopropanol (8.8 weight %) and stuccoed in sequence by about 100 mesh zircon, 60 mesh Mulgrain M-47 mullite and the balance being stuccoed by about 25 mesh Mulgrain M-47 mullite. The shells were steam autoclaved and then fired as described above. - Also compared is the weight of thick-wall shell (i.e., 6 mm (.25 inch) wall thickness) used previously for the parts and the weight of the thin-walled shell (i.e., about 2.5 mm (.10 inch) wall thickness) of the invention.
Table I SHELL LOADING AND FINAL REFRACTORY WEIGHT Part Std. Shell Loading pce/shell Thin Shell Loading pce/shell Std. Shell Weight Thin Shell Weight change % g (oz)/pce g (oz)/pce change % Rocker arm 8 ar X 13 hi 12 ar X 16 hi85 178 6.3 42.5 1.5 76 104 / shell 192 / shell Window latch 12 ar X 8 hi 14 ar X 10 hi46 190 6.7 42.5 1.5 63 96 / shell 140 / shell Cleat 10 ar 24 hi 12 ar X 26 hi 30 79.4 2.8 36.8 1.3 54 204 / shell 312 / shell Note: "ar" is # of mold cavities around riser and
"hi" is # of levels of mold cavities along riser
"oz/pce" is ounces per piece - From Table I, it is apparent that the thinner shell molds of the invention significantly increase the loading factor (i.e., parts castable per mold) and significantly reduce the amount of refractory material needed to form the fired shell. All this is achieved while also achieving equivalent or better values for mold distortion and damage during the steam autoclaving operation.
- In accordance with one embodiment of the invention, molten metal is differential pressure, countergravity cast into the thin shell mold 30 (after the shell is fired at about 982°C (1800°F)) as illustrated in Fig. 4. In particular, the
thin shell mold 30 is supported in a loose refractoryparticulate support media 60 itself contained in avacuum housing 70. Thevacuum housing 70 includes abottom support wall 72, anupstanding side wall 73 and a movabletop end wall 74 defining therewithin avacuum chamber 76. Thebottom wall 72 and theupstanding side wall 73 are formed of gas impermeable material, such as metal, while the movabletop end wall 74 comprises a gas permeable (porous)plate 75 having avacuum plenum 77 connected thereto to define avacuum chamber 78 above (outside) the gaspermeable plate 75. Thevacuum chamber 78 is connected to a source of vacuum, such as avacuum pump 80, byconduit 82. The movabletop end wall 74 includes aperipheral seal 84 that sealingly engages the interior of theupstanding side wall 73 to allow movement of thetop end wall 74 relative to theside wall 73 while maintaining a vacuum seal therebetween. - In assembly of the components shown in Fig. 4 to form the
casting apparatus 100, Fig. 4, aceramic fill tube 90 disposed inhousing 70 and providing a lower molten metal inlet to themold cavities 36 via theriser 37 and therespective ingates 38 is sealingly engaged to the frusto-conical collar 18 as the mold is placed thereon. Arefractory cap 20 is placed atop the shell mold to close off the upper end of theriser 37. The loose refractory particulate support media 60 (e.g., loose foundry silica sand of about 60 mesh) is introduced into thevacuum chamber 76 about the firedshell 30 while thehousing 70 is vibrated to aid in settling of thesupport media 60 in thechamber 76 about theshell 30. The movabletop end wall 74 is then positioned in the open upper end of thehousing 70 with theperipheral seal 84 sealingly engaging theupstanding side wall 73 and with the inner side of the gaspermeable plate 75 facing and in contact with thesupport media 60, Fig. 4. - After assembly, the
casting apparatus 100 is positioned above a source 102 (e.g., a pool) of themolten metal 104 to be cast. Typically, themolten metal 104 is contained in acasting vessel 106. A vacuum is then drawn in thevacuum chamber 78 of thevacuum bell 77 and hence in thevacuum chamber 76 through the gaspermeable plate 75 by actuation of thevacuum pump 80. Evacuation of thechamber 76, in turn, evacuates themold cavities 36 through the thin gas permeable shell wall. The level of vacuum inchamber 76 is selected sufficient to draw themolten metal 104 upwardly from thepool 102 into themold cavities 36 when thefill tube 90 is immersed in themolten metal 104 as shown in Fig. 4. - When the vacuum is drawn in
vacuum chambers top end wall 74 is subjected to atmospheric (or ambient) pressure on the side thereof external of theperipheral seal 84 while the inner side of theplate 75 is subjected to a relative vacuum. This pressure differential across thetop end wall 74 causes thetop wall 74 to move downwardly relatively toside wall 73 and causes theplate 75 to exert sufficient pressure on thesupport media 60 so as to compress or rigidize thesupport media 60 about theshell 30 to support it against casting stresses. Thus, while themold cavities 36 are evacuated to draw the molten metal upwardly from thepool 102, a pressure is applied concurrently by theplate 75 to compress thesupport media 60 about theshell 30 to support it against casting stresses. The amount of pressure applied by theplate 75 to compress thesupport media 60 can be controlled by controlling the level of vacuum established in thevacuum chamber 76. - As is apparent from Fig. 4, the
molten metal 104 will be drawn upwardly through thefill tube 90 through theriser 37 and into themold cavities 36 via thelateral ingates 38. Themolten metal 104 is thereby vacuum countergravity cast into themold cavities 36. - When the relative vacuum is established in
vacuum chamber riser 37 will be closest to the highest vacuum level inchamber 78. Moreover, it will be apparent that thesupport media 60 will act to reduce the vacuum level existing external of theshell 30 in proximity to its bottom. As a result, the stress imposed on the lower portions of theshell 30 by the combination of the internal metallostatic head and the external vacuum about theshell mold 30 is reduced in accordance with the principles of US-A-4,791,977. This reduction in stress in conjunction with support of theshell 30 by thesupport media 60 permits countergravity casting of the high temperaturemolten metal 104 into the thin shell 30 (having a wall thickness not exceeding about .12 inch) without harmful mold wall movement and molten metal penetration into the mold wall. - Should any small opening or similar defect be present in the
shell 30, the surroundingsupport media 60 also aids in preventing themolten metal 104 from leaking through the defect and, in any event, confines any leakage in proximity to theshell 30 to prevent damage to the casting apparatus, permitting vacuum to be held until the castings solidify. - Once the
molten metal 104 solidifies in themold cavities 36, the castingassembly 100 is moved upwardly to remove thefill tube 90 from the molten metal pool. Thetop wall 74 of thehousing 70 is then removed at an unload station (not shown) to allow removal of thesupport media 60 and the metal-filledshell 30 from thevacuum chamber 76. After cooling, thesupport media 60 can be recycled for reuse in casting anothershell 30. After removal from thevacuum chamber 76, the metal-filledshell 30 is allowed to cool to ambient. Theshell 30 is easily removed from the solidified casting by virtue of its thin wall thickness. For example, cooling of the metal-filledshell 30 often causes theshell 30 to simply pop off the casting due to thermal stresses imposed on the shell during cooling. In general, considerably less time is required to remove thethin shell 30 than to remove shell molds having thicker wall thicknesses heretofore used. - Referring to Fig. 5, a casting apparatus 100' of another embodiment of the invention is illustrated. In Fig. 5, like features of Fig. 4 are represented by like reference numerals primed. The casting apparatus 100' of Fig. 5 differs from the
casting apparatus 100 of Fig. 4 in using an annular vacuum bell 110' about thehousing 70' and a flexible, gas impermeable membrane 112' sealingly disposed on the open upper end of thehousing 70' (providing a movable housing top end wall) for applying a pressure to the support media 60' when thehousing 70' is evacuated. The vacuum plenum 110' defines an annular vacuum chamber 114' about the vacuum chamber 76' of thehousing 70' and is interconnected thereto by an annular gas permeable (porous) side wall housing section 116'. - As will be apparent, when the vacuum chamber 114' is evacuated (via conduit 118'), the vacuum chamber 76' and the mold cavities 36' of the shell 30' are, in turn, evacuated.
- When the vacuum is drawn in the vacuum chamber 76', the flexible, gas impermeable membrane 112' is subjected to atmospheric pressure on the outside surface 112a' and to the relative vacuum on the inside surface 112b', causing the membrane 112' to compress the loose refractory particulate support media 60' about the thin shell mold 30' to support it against casting stresses in the manner described hereinabove with respect to Fig. 4 as the molten metal is urged upwardly from the underlying pool through the fill tube 90' and the riser 37' and into the mold cavities 36' via the ingates 38'. In other respects, the embodiment of Fig. 5 functions and offers advantages described above for the embodiment of Fig. 4.
- Referring to Fig. 6, a casting apparatus 100'' of still another embodiment of the invention is illustrated wherein like features of Fig. 4 are represented by like reference numerals double primed. The embodiment of Fig. 6 differs from that of Fig. 4 in using one or more annular fluid pressurizable bladders 120'' (one shown) disposed in contact with the refractory
particulate support media 60" in thehousing 70'' to exert a pressure on the support media 60'' to compress it about the thin shell mold 30'' when the mold cavities 36'' are evacuated during countergravity casting. Thehousing 70'' includes a non-movable top end wall 74'' which comprises a gas permeable plate 75'' sealed to the top of thehousing 70'' by seal 84'' and a vacuum bell 77'' connected to the plate 75''. The vacuum chamber 78'' of the bell 77'' overlies the gaspermeable portion 75a'' of the plate 75'' so as to evacuate the vacuum chamber 76'' of thehousing 70'' by a means of vacuum pump 80'' communicating thereto via conduit 82''. - After the top end wall 74'' is sealed to the
housing 70'' and the chambers 76'',78'' are evacuated, the bladder 120'' is pressurized by a suitable gas supply 121'', such as compressed air, through suitable gas supply pipes 122''. Pressurization of the bladder 120'' exerts a pressure on the refractory particulate support media 60'' to compress it about the shell 30'' to support it against casting stresses in the same manner as described hereinabove for the preceding embodiments. In other respects, the embodiment of Fig. 6 is similar in function to the preceding embodiments of Figs. 4 and 5. - While the invention has been described in terms of specific embodiments thereof, it is not intended to be limited thereto but only to the extent set forth hereafter in the claims which follow.
Claims (19)
- A method of casting molten metal comprising the following successively performed steps:(a) forming an expendable pattern (10) of an article to be cast, said pattern comprising a meltable material that expands upon heating,(b) investing the pattern (10) with particulate refractory mold material (22) in multiple layers so as to form a refractory shell (30) thereabout,(c) heating the invested pattern (40) to remove the pattern (10) from the shell (30) to leave a mold cavity (36) therein,(d) disposing a refractory particulate support media (60) about the thin shell (30) with the mold cavity (36) communicated to a molten metal inlet (18, 18a) disposed external of the support media,(e) evacuating the mold cavity (36),(f) casting the molten metal into the evacuated mold cavity (36) while the molten metal inlet is communicated to a source of the molten metal and the shell (30) is supported in the support media (60),said casting method being characterized
in that it comprises a method of countergravity casting a molten metal upwardly into the evacuated mold cavity (36) while the molten metal inlet (18, 18a) is communicated to an underlying source of the molten metal,
in that in said shell forming step (b) application of said layers of mold material is controlled so as to form a thin refractory shell (30) having a wall thickness not exceeding about 3 mm (0.12 inch) and
in that it further comprises applying such a pressure to the support media (60) concurrently with the evacuation of the mold cavity (36) as to compress the support media about the thin shell (30) to support said shell against casting stresses. - The method of claim 1 wherein the thin, layered shell (30) is so formed as to exhibit a gas permeability more than twice that exhibited by a like shell having twice the wall thickness.
- The method of claim 2 wherein the shell (30) is so formed as to exhibit a gas permeability of at least three times that exhibited by said like shell.
- The method of claim 1 wherein in step (c) the invested pattern (10) is steam autoclaved to remove the pattern from the thin shell (30) to leave a mold cavity (36) therein.
- The method of claim 1 wherein in step (d), the shell (30) is supported in the support media (60) inside a vacuum housing (70) and a pressure transmitting means (74) is moved relative to the vacuum housing (70) and the support media (60) when the housing is evacuated to exert said pressure on the support media.
- The method of claim 5 wherein the pressure transmitting means comprises a movable wall (74) of the housing (70) for pressing on the support media (60).
- The method of claim 6 wherein the wall (74) is subjected to a relative vacuum on the inner side thereof and ambient pressure on the outer side thereof.
- The method of claim 5 wherein the pressure transmitting means comprises a bladder (120'') disposed in contact with the support media in the housing (70'') and pressurized to compress the support media when the mold cavity (36'') is evacuated.
- The method of claim 1 wherein the expendable pattern (10) comprises wax.
- The method of claim 1 wherein the expendable pattern (10) comprises urea.
- The method of claim 1 wherein said step (d) of disposing said support media (60) about the shell comprises surrounding the thin shell with a refractory particulate support media contained in a vacuum chamber with said mold cavity communicated to a lower molten metal inlet (18, 18a) disposed external of said vacuum chamber.
- Apparatus for countergravity casting of molten metal, comprising:(a) refractory particulate support media (60) disposed in a housing (70),(b) a refractory investment shell (30) having a mold cavity (36) defined by a mold wall disposed in the support media (60),(c) means (78, 80, 82) for evacuating the mold cavity (36),(d) a molten metal inlet (90) disposed external of the support media (60) for communicating the mold cavity (36) and a source (102) of the molten metal, when the mold cavity (36) is evacuated so as to urge the molten metal into the evacuated mold cavity (36),said apparatus being characterized in that said shell has a mold wall thickness not exceeding about 3 mm (0.12 inch),
that said molten metal inlet is a lower inlet (90) for communicating the mold cavity (36) and an underlying source (102) of the molten metal and that means (74) are provided for applying a pressure to the support media (60) concurrently with the evacuation of the mold cavity (36), said pressure being such as to compress the support media (60) about the shell (30) to support it against casting stresses. - The apparatus of claim 12 wherein said molten metal inlet comprises a fill tube (90) extending from the shell (30) external of the support media (60).
- The apparatus of claim 12 wherein the means for applying pressure to the support media (60) comprises a movable wall (74) of said housing (70), said movable wall (74) being subjected to such a differential pressure when a chamber (76) in said housing (70) is evacuated as to cause said wall (74) to move relative to the housing (70) and support media (60) to compress the support media about the shell (30).
- The apparatus of claim 14 wherein the movable wall comprises a gas permeable end wall (74) of the housing, said end wall having an inner side for contacting the support media (60) and a vacuum bell (77) overlying the outer side thereof, said vacuum bell (77) being evacuable on the inside to evacuate the chamber (76) through the gas permeable end wall (74) and being subjected to ambient pressure on the outside thereof such that said wall (74) moves relative to the housing (70) to press on the support media (60) when said chamber (76) is evacuated.
- The apparatus of claim 14 wherein the movable wall comprises a gas impermeable, flexible end wall (112') of the housing (70').
- The apparatus of claim 12 wherein the means for applying pressure to the support media comprise a pressurizable bladder (120'') disposed in contact with the support media (60'') in the chamber (76'').
- The apparatus of claim 12 wherein the refractory particulate support media comprises loose foundry sand.
- The apparatus of claim 12 wherein said housing (70) has a vacuum chamber (76);
wherein said loose refractory particulate support media (60) is disposed in the chamber (76) so as to be compressed about said shell (30); and
wherein said lower molten metal inlet (90) is disposed external of the vacuum chamber (76).
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US57931990A | 1990-09-06 | 1990-09-06 | |
US579319 | 1990-09-06 |
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EP0474078A1 EP0474078A1 (en) | 1992-03-11 |
EP0474078B1 true EP0474078B1 (en) | 1995-11-29 |
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Application Number | Title | Priority Date | Filing Date |
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EP19910114237 Expired - Lifetime EP0474078B1 (en) | 1990-09-06 | 1991-08-24 | Countergravity casting using particulate supported thin walled investment shell mold |
Country Status (10)
Country | Link |
---|---|
EP (1) | EP0474078B1 (en) |
JP (1) | JPH07110404B2 (en) |
CN (1) | CN1047547C (en) |
AU (1) | AU635858B2 (en) |
BR (1) | BR9103830A (en) |
CA (1) | CA2049228C (en) |
DE (1) | DE69114954T2 (en) |
MX (1) | MX173895B (en) |
RU (1) | RU2039629C1 (en) |
YU (1) | YU47814B (en) |
Cited By (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US9833833B2 (en) | 2013-03-14 | 2017-12-05 | Hitchiner Manufacturing Co., Inc. | Refractory mold and method of making |
Families Citing this family (19)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US5303762A (en) * | 1992-07-17 | 1994-04-19 | Hitchiner Manufacturing Co., Inc. | Countergravity casting apparatus and method |
JPH0818122B2 (en) * | 1993-02-02 | 1996-02-28 | 第一鋳造株式会社 | Vacuum shell mold casting method |
EP0640420B1 (en) * | 1993-03-12 | 2000-06-07 | Hitachi Metals, Ltd. | Vacuum suction casting apparatus |
GB9724568D0 (en) * | 1997-11-19 | 1998-01-21 | Castings Dev Centre The | Investment casting |
US8312913B2 (en) | 2005-02-22 | 2012-11-20 | Milwaukee School Of Engineering | Casting process |
CN101386066B (en) * | 2008-10-27 | 2010-06-09 | 秦中林 | Method for casting the iron and steel casting on extrusion machine |
CN101850401A (en) * | 2010-05-31 | 2010-10-06 | 上虞新达精密铸造有限公司 | Fired mold and precision casting process utilizing same |
CN102806314A (en) * | 2012-09-03 | 2012-12-05 | 贵州安吉航空精密铸造有限责任公司 | Casting method for aluminum alloy thin-wall fine-hole casting |
US9486852B2 (en) | 2013-03-14 | 2016-11-08 | Hitchiner Manufacturing Co., Inc. | Radial pattern assembly |
US9481029B2 (en) | 2013-03-14 | 2016-11-01 | Hitchiner Manufacturing Co., Inc. | Method of making a radial pattern assembly |
US8931544B2 (en) * | 2013-03-15 | 2015-01-13 | Metal Casting Technology, Inc. | Refractory mold |
KR101367200B1 (en) * | 2013-05-08 | 2014-02-26 | 지정욱 | Process for duplex casting and process for duplex casting apparatus thereof |
CN104399891A (en) * | 2014-11-27 | 2015-03-11 | 温州兰理工科技园有限公司 | Casting method combining investment precise shell-making process with vacuum suction molding technology |
CN104972066A (en) * | 2015-06-15 | 2015-10-14 | 江苏万恒铸业有限公司 | Thin-shell sand culturing casting simple technology |
CN106513635B (en) * | 2016-12-16 | 2019-02-05 | 上海华培动力科技股份有限公司 | Double-layer hollow cylinder fusible pattern formwork structure for high-temperature alloy process for suction casting |
CN109719277A (en) * | 2017-10-30 | 2019-05-07 | 科华控股股份有限公司 | The anti-gravity feeding shell structure of heat resisting steel volute process for suction casting |
CN109465399B (en) * | 2018-12-26 | 2023-11-21 | 广东富华铸锻有限公司 | Gating system with speed reducer shell sand core structure |
JP6915206B2 (en) * | 2019-08-26 | 2021-08-04 | 呉政寛 | Molten forging method for thin-walled shell molds |
CN114309474A (en) * | 2022-03-14 | 2022-04-12 | 成都航宇超合金技术有限公司 | Investment casting process of small and medium-sized single crystal turbine blades |
Family Cites Families (7)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
AT274263B (en) * | 1964-07-09 | 1969-09-10 | Dynamit Nobel Ag | Process for the production of precision casting molds |
US3900064A (en) * | 1972-12-04 | 1975-08-19 | Hitchiner Manufacturing Co | Metal casting |
IT1121553B (en) * | 1979-06-01 | 1986-04-02 | Cattaneo Leopoldo | ELASTIC BANDS AND / OR SCRAPER FOR THERMAL ENGINES AND / OR FOR SEALS OF ANY KIND, WITH PERIPHERAL RIGIDITY AND DEFORMATION IN HOT OPERATION CONTROLLED THROUGH CARVINGS, HOLES, LIGHTENINGS AND CRACKS PRACTICED ON THE SURFACES OF THE LEVEL OF THE LEVEL OF THE FLEE LEAD OF THE PASTA LEAD. FOR THEIR PRODUCTION |
US4340108A (en) * | 1979-09-12 | 1982-07-20 | Hitchiner Manufacturing Co., Inc. | Method of casting metal in sand mold using reduced pressure |
US4589466A (en) * | 1984-02-27 | 1986-05-20 | Hitchiner Manufacturing Co., Inc. | Metal casting |
US4791977A (en) * | 1987-05-07 | 1988-12-20 | Metal Casting Technology, Inc. | Countergravity metal casting apparatus and process |
IN170880B (en) * | 1987-05-07 | 1992-06-06 | Metal Casting Tech |
-
1991
- 1991-08-15 CA CA 2049228 patent/CA2049228C/en not_active Expired - Lifetime
- 1991-08-16 AU AU82495/91A patent/AU635858B2/en not_active Expired
- 1991-08-24 EP EP19910114237 patent/EP0474078B1/en not_active Expired - Lifetime
- 1991-08-24 DE DE1991614954 patent/DE69114954T2/en not_active Expired - Lifetime
- 1991-09-04 BR BR9103830A patent/BR9103830A/en not_active IP Right Cessation
- 1991-09-05 MX MX9100964A patent/MX173895B/en unknown
- 1991-09-05 YU YU149191A patent/YU47814B/en unknown
- 1991-09-05 CN CN91108800A patent/CN1047547C/en not_active Expired - Lifetime
- 1991-09-05 JP JP25280891A patent/JPH07110404B2/en not_active Expired - Lifetime
- 1991-09-05 RU SU5001482 patent/RU2039629C1/en active
Cited By (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US9833833B2 (en) | 2013-03-14 | 2017-12-05 | Hitchiner Manufacturing Co., Inc. | Refractory mold and method of making |
Also Published As
Publication number | Publication date |
---|---|
RU2039629C1 (en) | 1995-07-20 |
AU8249591A (en) | 1992-03-12 |
YU47814B (en) | 1996-01-09 |
YU149191A (en) | 1994-06-24 |
CN1059485A (en) | 1992-03-18 |
CN1047547C (en) | 1999-12-22 |
JPH07110404B2 (en) | 1995-11-29 |
JPH04270024A (en) | 1992-09-25 |
CA2049228A1 (en) | 1992-03-07 |
EP0474078A1 (en) | 1992-03-11 |
BR9103830A (en) | 1992-05-19 |
AU635858B2 (en) | 1993-04-01 |
DE69114954T2 (en) | 1996-07-18 |
CA2049228C (en) | 1996-10-15 |
DE69114954D1 (en) | 1996-01-11 |
MX173895B (en) | 1994-04-07 |
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