DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side, section view of a lost-foam mold with embedded pattern and chill according to one embodiment of the present invention.
FIG. 2 is a view in the direction 2-2 of FIG. 1.
FIG. 3 is sectional view of a low pressure, counter gravity casting vessel with overlying lost-foam mold pattern and chill sets in accordance with another embodiment of the present invention.
FIG. 4 are photographs of certain surface areas of lost-foam aluminum castings made (a) without a chill, (b) with a chill engaging the pattern's surface, and (c) with a chill spaced from the pattern's surface by a gap in accordance with the present invention.
FIG. 5 are photomicrographs of the microstructure of regions of certain aluminum lost-foam castings made (a) without a chill, and (b) with a chill spaced from the casting's pattern by a gap in accordance with the present invention.
FIG. 6 is a section view of a lost-foam mold with embedded pattern and chill according to still another embodiment of the present invention.
DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION
FIGS. 1 and 2 depict a bottom-fed, gravity-cast lost-foam casting mold including a hydrolysable polymeric foam (e.g. EPS) pattern 2 coated with a permeable refractory coating 4, and spaced from a metal chill 6 by means of a foam spacer 8 that is molded integrally with the pattern 2 at the time the pattern is molded, and takes the form of a continuous ridge, or rib, projecting from and circumscribing a selected surface 12 of the pattern 2. The chill 6 is preferably perimeter-glued to the spacer 8 using a vaporizable glue, ala Runts Jr. et al. supra. The pattern-chill assembly is embedded in a bed of refractory particles 20 (e.g. sand) contained in a metal flask 22 using fluidization and compaction techniques well known to lost-foam process practitioners. The chill 6 has a cooling face 10 that confronts the selected surface 12 of the pattern 2 across a gap 14 therebetween. The gap is at least 0.5 mm wide, and is adapted to be filled with melt in such a way that the advancing metal front therein displaces the collapsing foam pattern and keeps pyrolysis products away from the cooling face 10 of the chill 6. In this embodiment, a vertical, ceramic down-sprue 15 supplies melt 16 through an inlet 18 directly to the gap 14, and thence into the foam pattern 2. The chill 6 is arranged vertically in the mold, and the melt front moves upwardly through the gap 14 so as to sweep any pyrolysis products ahead of it, and out of the gap 14. It then advances laterally into the pattern 2. Once the gap is filled with metal, a barrier is formed that prevents pyrolysis products from reaching, and being trapped against, the cooling face 10 by the melt.
FIG. 3 depicts a counter-gravity, low-pressure lost-foam casting process wherein molten metal 24, from an underlying crucible 26, is forced upwardly, by gas pressure, into a lost-foam mold 28 overlying the crucible 26. The mold 28 contains two pattern-chill sets 52 and 54. Set 54 has a gap 56 between the chill 39 and the confronting face 58 of the foam pattern 37, according to the present invention. Set 52 has no such gap, and the foam pattern 46 abuts the chill 48. More specifically, a pressurizable vessel 30 is sealed closed by a cover 32 and contains a heated crucible 26 filled with melt 24. A flask 40 is supported above the crucible 26 by the cover 32. A filler-tube 38 extends from beneath the surface 36 of the melt 24 in the crucible 26 upwardly through the cover 32 and into sealing engagement with an opening 50 in the bottom of flask 40, and communicates with a gating and runner system 42 which connects to the pattern-chill sets 52 and 54. An inlet pipe 34 to the vessel 30 allows pressurized gas (e.g. nitrogen) to be admitted to the vessel 30 to pressurize the vessel 30, and thereby apply pressure on the upper surface 36 of the melt 24. Pressurizing the vessel 30 causes the melt to rise up the filler-tube 38, through the gating and runner system 42, and into the molding cavities occupied by the foam patterns 37 and 46 in the unbonded sand mold 44.
FIGS. 4 and 5 are photographs and photomicrographs respectively of A356 aluminum castings simultaneously cast, with and without chills, by the low-pressure technique described in connection with FIG. 3.
EPS patterns 37 and 46 were 15 cm wide by 20 cm high by 12 mm thick. The chills 39 and 48 were 15 cm wide by 20 cm high by 5 cm thick copper. The gap 33 was 4 mm. The chills were perimeter-glued to the patterns, as described above in connection with FIG. 1. The A356 melt was cast using a programmed, variable, nitrogen-imposed pressure in the vessel 30 at a temperature of 775° C. in the crucible 26.
FIG. 4 shows photographs of the surface area of A356 Al castings made (a) without a chill, (b) with a chill whose cooling face contacted the pattern, and (c) with a chill whose cooling face was spaced from the pattern by a 4 mm gap and filled with melt in accordance with the present invention. FIG. 4 (b) shows the rough surface of the casting made with the chill contacting the pattern and resulting from the melt trapping pyrolysis products against the cooling face of the chill. In sharp contrast, FIG. 4(c) shows the relatively smooth surface of the casting made with a chill spaced from the pattern by a gap in accordance with the present invention.
Similarly, FIG. 5 shows (a) the coarse microstructure of an A356 Al casting made without a chill, and (b) the refined microstructure of a casting made with a chill and gap according to the present invention.
FIG. 6 depicts another embodiment wherein the gate 60 is remote from the chill 62, but flow communicates directly with the gap 64 by means of lighteners 66. In this embodiment, the chill 62 is oriented horizontally in the mold beneath the foam pattern 70, so that the molten metal 72 first begins to spread out over the cooling face 68 of the chill 62 before it rises in the gap 64 into contact with the foam pattern 70. The metal-filled gap prevents any pyrolysis products from reaching the cooling face 68, while the metal front pushes pyrolysis products further away from the cooling face 68.
While the invention has been described in terms of certain specific embodiments thereof, it is not intended to be limited thereto, but rather only to the extent set forth hereafter in the claims which follow.
Patent Application
20070295469
