Note: Descriptions are shown in the official language in which they were submitted.
~ ~ 20492~8
COUNTERGRAVITY CASTING USING PARTICULATE
SUPPORTED THIN WALLED INVESTMENT SHELL MOLD
Field of the Invention
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.
Background Of The Invention
Vacuum-assisted countergravity casting processes
using gas permeable investment shell molds are
described in the Chandley U.S. Patents 3,900,064;
4,340.108; 4,532,976; 4,S89,466 and 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
P-306 Metal Casting 2
20~92~8
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 275F to about 350F 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
P-306 Metal Casting 3
2049228
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 1/4 inch to
this end.
Aforementioned U.S. Patent 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
P-306 Metal Casting 4
20~9~28
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.
It is another object of the present invention to
- P-306 Metal Casting 5
20~92~8
provide 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 still another object of the invention to
provide 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 object of the invention to
provide 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 object of the invention to
provide 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
P-306 Metal Casting 6
2049228
therefrom.
Summary Of The Invention
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 .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
P-306 Metal Casting 7
20~g228
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 .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 .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.
P-306 Metal Casting 8
20~9~28
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 .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
- 20~9228
P-306 Metal Casting 9
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.
Brief Description Of The 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
P-306 Metal Casting 10
20~9228
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.
P-306 Metal Casting 11
2049~2~
Detailed Description 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-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.
Typically, 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.
P-306 Metal Casting 12
~0492:~8
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 130F to about
150F. Importantly, wax viscosity must be selected to
avoid shell cracking during the pattern operation
(e.g., wax viscosity at 170F should be less than 1300
centipoise). Urea may also be useful as a pattern
material and melts in the range of about 235F to
about 265F.
It is not necessary in practicing the present
invention that 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.
Referring now to Fig. 2, 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
20~9228
P-306 Metal Casting 13
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 a multi-layered shell 30
of desired wall thickness t about the mold
cavity-forming portions 14 is built up.
In accordance with the present invention, the
P-306 Metal Casting 14
20~228
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 .12 inch about the mold cavity-forming
S portions 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
.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. In particular, 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. As
is apparent, 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. At this stage of
processing, the riser 37 is open at the lower and
P-306 Metal Casting 15
20~Y2~8
upper ends.
During the steam autoclaving operation, the
invested pattern assembly 40, Fig. 2, is subjected to
steam at a temperature of about 275F to about 350F
(steam pressure of about 80 psi to about 110 psi) for
a time sufficient to melt the pattern assembly 10 out
of the refractory 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 permeable refractory shell 30. As
will be explained hereinbelow, 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. Over time, 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.
As mentioned hereinabove, the wall thickness of
P-306 Metal Casting 16
20 4922~
the refractory shell 30 is controlled in accordance
with the invention so as not to exceed about .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 1900F
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 1800F having a
wall thickness of about .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
lS .25 inch and comprising eight refractory layers). In
particular, the gas permeability of the fired
refractory shell 30 of .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 .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
- P-306 Metal Casting 17
20~9228
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 .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 the pattern
assembly 10 relative to the refractory 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 the
high permeability shell 30 and resulting faster
heating of the pattern assembly 10.
~ P-306 Metal Casting 18
~049~28
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 7S% 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 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).
After steam autoclaving the shell is fired at
about 1800F 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
- 20492~8
P-306 Metal Casting 19
using thick-walled shells (i.e., .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., .25 inch wall thickness) used previously for
the parts and the weight of the thin-walled shell
(i.e., about .10 inch wall thickness) of the
invention.
- 204922~P-306 Metal Casting 20
Table I
SHELL LOADING AND FINAL REFRAcToRr ~EIGHT
Std. ShellThin ShellStd. Shell Thin Shell
LoadingLoading ~eight ~eight
Part pce/shellDce/shell chance X oz/rxe oZ/DCe chanse X
Rocker arm8 ar X 13 hi 12 ar X 16 hi 6.3 1.5 76
104 / shell 192 / shell 85
~indo~ latch12 ar X 8 hi 14 ar X 10 hi 6.7 1.5 63
96 / shell 140 / shell 46
Cleat 10 ar 24 hi 12 ar X 26 hi 2.8 1.3 54
204 / shell 312 / shell 30
Note: "ar" is # of mold cavities around riser and
"hi~' is # of levels of mold cavities along riser
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 1800F) as illustrated in
Fig. 4. In particular, the thin shell mold 30 is
supported in a loose refractory particulate support
P-306 Metal Casting 21
20~9228
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.
In assembly of the components shown in Fig.
4 to form the casting apparatus 100, Fig. 4, 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
P-306 Metal Casting 22
20~9228
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
S about 60 mesh) is introduced into the vacuum chamber
76 about the fired shell 30 while 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.
After assembly, the casting apparatus 100 is
positioned above a source 102 (e.g., a pool) of the
molten metal 104 to be cast. Typically, 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, in
turn, evacuates the mold cavities 36 through the thin
P-306 Metal Casting 23
2~9~28
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.
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.
Thus, while the mold cavities 36 are evacuated to draw
the molten metal upwardly from the pool 102, 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
P-306 Metal Casting 24
2~492~8
level of vacuum established in the vacuum chamber 76.
As is apparent from Fig. 4, 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.
When the relative vacuum is established in
vacuum chamber 76,78, it is apparent that upper end of
the riser 37 will be closest to the highest vacuum
level in chamber 78. Moreover, it will be apparent
that the support media 60 will act to reduce the
vacuum level existing external of the shell 30 in
proximity to its bottom. As a result, the stress
imposed on the lower portions of the shell 30 by the
combination of the internal metallostatic head and the
external vacuum about the shell mold 30 is reduced in
accordance with the principles of U.S. Patent
4,791,977. 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
P-306 Metal Casting 25 2~4;922~
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 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.
Once the molten metal 104 solidifies in the mold
cavities 36, 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. After cooling,
the support media 60 can be recycled for reuse in
casting another shell 30. After removal from the
vacuum chamber 76, 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
- P-306 Metal Casting 26 2 0 4 9 ~ 2 8
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 thicknessés 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 llO' 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'.
P-306 Metal Casting 27
204922~
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.
P-306 Metal Casting 28 2 0 49 ~ 2 8
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
5 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
10 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
15 housing 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,
20 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"
P-306 Metal Casting 29
20~22g
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
S 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 rather only to the extent set
forth hereafter in the claims which follow.
