[0001] 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.
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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 to claim 12.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] 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.
[0015] 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.
[0016] 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.
[0017] 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.
[0018] 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.
[0019] 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.
[0020] The aforementioned objects and advantages of the present invention will become more
readily apparent from the following detailed description and drawings.
[0021] Figure 1 is a side elevation of a pattern assembly.
[0022] Figure 2 is a sectioned, side elevation of the pattern assembly after investing with
particulate mold material.
[0023] Figure 3 is a sectioned, side elevation of the thin shell mold after the pattern
assembly is removed by steam autoclaving
[0024] 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.
[0025] Figure 5 is a sectioned side elevation of a countergravity casting apparatus in accordance
with another embodiment of the invention.
[0026] Figure 6 is a sectioned side elevation of a countergravity casting apparatus in accordance
with still another embodiment of the invention.
[0027] 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.
[0028] A refractory frusto-conical collar 18 is attached (e.g., wax welded or adhered) to
the lower end of the riser-forming portion 12.
[0029] 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).
[0030] 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.
[0031] 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 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.
[0032] 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.
[0033] 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-forming
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 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.
[0034] 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 upper ends.
[0035] 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 the pattern assembly 10 out of the refractory shell 30.
[0036] 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.
[0037] 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 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.
[0038] 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.
[0039] 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 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.
[0040] 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.
[0041] 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).
[0042] After steam autoclaving the shell is fired at about 1800°F for 90 minutes.
[0043] 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.
[0044] 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 hi |
85 |
178 |
6.3 |
42.5 |
1.5 |
76 |
104 / shell |
192 / shell |
Window latch |
12 ar X 8 hi |
14 ar X 10 hi |
46 |
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 |
[0045] 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.
[0046] 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 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.
[0047] 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 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) 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.
[0048] 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 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.
[0049] 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 level of vacuum
established in the vacuum chamber 76.
[0050] 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.
[0051] 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 US-A-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 thickness
not exceeding about .12 inch) without harmful mold wall movement and molten metal
penetration into the mold wall.
[0052] 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.
[0053] 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 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.
[0054] 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 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'.
[0055] 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.
[0056] 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.
[0057] 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 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''.
[0058] 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.
[0059] 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.
1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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).
7. 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.
8. 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.
9. The method of claim 1 wherein the expendable pattern (10) comprises wax.
10. The method of claim 1 wherein the expendable pattern (10) comprises urea.
11. 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.
12. 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.
13. 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).
14. 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).
15. 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.
16. The apparatus of claim 14 wherein the movable wall comprises a gas impermeable, flexible
end wall (112') of the housing (70').
17. 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'').
18. The apparatus of claim 12 wherein the refractory particulate support media comprises
loose foundry sand.
19. 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).
1. Verfahren zum Gießen von geschmolzenem Metall, welches die folgenden, nacheinander
durchgeführten Schritte umfaßt:
(a) es wird ein verlorenes Modell (10) eines zu gießenden Artikels hergestellt, wobei
das Modell ein schmelzbares Material umfaßt, welches sich beim Erhitzen dehnt,
(b) das Modell (10) wird mit einem partikelförmigen feuerfesten Formmaterial (22)
in mehreren Lagen umhüllt, um um das Modell einen feuerfesten Mantel (30) zu bilden,
(c) das umhüllte Modell (40) wird erhitzt, um das Modell (10) aus dem Mantel (30)
zu entfernen und in diesem einen Hohlraum (36) zurückzulassen,
(d) ein feuerfestes, partikelförmiges Stützmedium (60) wird rund um den dünnen Mantel
(30) mit dem Formhohlraum (36) angeordnet, der mit einem Einlaß (18, 18a) für geschmolzenes
Metall kommuniziert, welches außerhalb des Stützmediums vorgesehen ist,
(e) der Formhohlraum (36) wird evakuiert,
(f) das geschmolzene Metall wird in den evakuierten Formhohlraum (36) gegossen, während
der Einlaß für das geschmolzene Metall mit einer Quelle für das geschmolzene Metall
kommuniziert und der Mantel (30) in dem Stützmedium (60) abgestützt ist,
wobei dieses Gießverfahren dadurch gekennzeichnet ist, daß es ein Verfahren des Steigendgusses
umfaßt, bei dem ein geschmolzenes Metall nach oben in den evakuierten Formhohlraum
(36) gegossen wird, während der Einlaß (18, 18a) für das geschmolzene Metall mit einer
darunter liegenden Quelle des geschmolzenen Metalls kommuniziert,
daß beim dem Schritt (b) zum Herstellen des Mantels das Anbringen der Schichten des
Formmaterials derart kontrolliert wird, daß ein dünner feuerfester Manteil (30) mit
einer Wandstärke von nicht mehr als 3 mm (0,12") gebildet wird, und
daß es (das Verfahren) ferner das Ausüben eines solchen Druckes auf das Stützmedium
(60) gleichzeitig mit der Evakuierung des Hohlraums (36) umfaßt, daß das Stützmedium
um den dünnen Mantel (30) zusammengepreßt wird, um den Mantel gegen Gießbelastungen
zu stützen.
2. Verfahren nach Anspruch 1, bei dem der dünne mehrlagige Mantel (30) so hergestellt
wird, daß er eine Gasdurchlässigkeit besitzt, die mehr als das Doppelte der Gasdurchlässigkeit
beträgt, die sich für einen (im übrigen) gleichen Mantel mit der doppelten Wandstärke
ergibt.
3. Verfahren nach Anspruch 2, bei dem der Mantel (30) so hergestellt wird, daß er eine
Gasdurchlässigkeit besitzt, die mindestens das Dreifache der Gasdurchlässigkeit für
den (im übrigen) gleichen Mantel beträgt.
4. Verfahren nach Anspruch 1, bei dem im Verlauf des Schrittes (c) das verlorene Modell
(10) in einem Autoklav mit Dampf behandelt wird, um das Modell aus dem dünnen Mantel
(30) zu entfernen und darin einen Formhohlraum (36) zurückzulassen.
5. Verfahren nach Anspruch 1, bei dem im Verlauf des Schrittes (d) der Mantel (30) in
dem Stützmedium (60) im Inneren eines Vakuumgehäuses (70) abgestützt wird und Druckübertragungseinrichtungen
(74) relativ zu dem Vakuumgehäuse (70) und dem Stützmedium (60) bewegt werden, wenn
das Gehäuse evakuiert wird, um den Druck auf das Stützmedium auszuüben.
6. Verfahren nach Anspruch 5, bei dem die Druckübertragungseinrichtungen eine bewegliche
Wand (74) des Gehäuses (70) zum Ausüben eines Druckes auf das Stützmedium (60) umfassen.
7. Verfahren nach Anspruch 6, bei dem die Wand (74) auf ihrer Innenseite einem relativen
Vakuum unterworfen ist und auf ihrer Außenseite dem Umgebungsdruck.
8. Verfahren nach Anspruch 5, bei dem die Druckübertragungseinrichtungen eine Blase (120'')
umfassen, die in dem Gehäuse (70'') in Kontakt mit dem Stützmedium angeordnet ist
und in der ein Druck erzeugt wird, um das Stützmedium zusammenzupressen, wenn der
Formhohlraum (36'') evakuiert wird.
9. Verfahren nach Anspruch 1, bei dem das verlorene Modell (10) Wachs umfaßt.
10. Verfahren nach Anspruch 1, bei dem das verlorene Modell (10) Harnstoff umfaßt.
11. Verfahren nach Anspruch 1, bei dem der Schritt (d) des Anordnens des Stützmediums
(60) um den Mantel herum das Umgeben des dünnen Mantels mit einem feuerfesten, partikelförmigen
Stützmedium umfaßt, welches in einer Vakuumkammer enthalten ist, wobei der Formhohlraum
mit einem unten liegenden Einlaß (18, 18a) für geschmolzenes Metall kommuniziert,
welcher außerhalb der Vakuumkammer angeordnet ist.
12. Vorrichtung für die Durchführung eines Steigendgusses von geschmolzenem Metall, umfassend:
(a) ein feuerfestes, partikelförmiges Stützmedium (60), welches in einem Gehäuse (70)
angeordnet ist,
(b) einen feuerfesten Einmalmantel (30) mit einem Formhohlraum (36), der durch eine
Formwand definiert ist, die in dem Stützmedium (60) angeordnet ist,
(c) Einrichtungen (78, 80, 82) zum Evakuieren des Formhohlraums (36),
(d) einen Einlaß (90) für geschmolzenes Metall, der außerhalb des Stützmediums (60)
angeordnet ist, um eine Verbindung zwischen dem Formhohlraum (36) und einer Quelle
(102) von geschmolzenem Metall herzustellen, wenn der Formhohlraum (36) evakuiert
wird, um das geschmolzene Metall in den evakuierten Formhohlraum (36) zu drücken,
wobei die Vorrichtung dadurch gekennzeichnet ist, daß der Mantel eine Form-Wandstärke
besitzt, die einen Wert von etwa 3 mm (0,012 ") nicht überschreitet,
daß der Einlaß für das geschmolzene Metall ein unten liegenden Einlaß (90) zum Herstellen
einer Verbindung zwischen dem Formhohlraum (36) und einer darunterliegenden Quelle
(102) des geschmolzenen Metalls ist und daß Einrichtungen (74) vorgesehen sind, um
gleichzeitig mit der Evakuierung des Formhohlraums (36) einen Druck auf das Stützmedium
(60) auszuüben, wobei dieser Druck einer solcher Druck ist, daß das Stützmedium (60)
um den Mantel (30) herum zusammengepreßt wird, um diesen gegen Gießbelastungen abzustützen.
13. Vorrichtung nach Anspruch 12, bei der der Einlaß für das geschmolzene Metall ein Füllrohr
(90) umfaßt, welches sich von dem Mantel (30) bis außerhalb des Stützmediums (60)
erstreckt.
14. Vorrichtung nach Anspruch 12, bei der die Einrichtungen zum Ausüben eines Druckes
auf das Stützmedium (60) eine bewegliche Wand (74) des Gehäuses (70) umfassen, wobei
die bewegliche Wand (74) beim Evakuieren einer Kammer (76) in dem Gehäuse (70) einem
solchen Differenzdruck ausgesetzt ist, daß die Wand (74) veranlaßt wird, sich relativ
zu dem Gehäuse (70) und dem Stützmedium (60) zu bewegen, um das Stützmedium rund um
den Mantel (30) zusammenzupressen.
15. Vorrichtung nach Anspruch 14, bei der die bewegliche Wand eine gasdurchlässige Stirnwand
(74) des Gehäuses umfaßt, wobei diese Stirnwand eine Innenseite zum Berühren des Stützmediums
(60) und einer Vakuumglocke (77) aufweist, die über der Außenseite desselben (des
Stützmediums) liegt, wobei die Vakuumglocke (77) auf der Innenseite evakuierbar ist,
um die Kammer (76) durch die gasdurchlässige Stirnwand (74) hindurch zu evakuieren
und auf ihrer Außenseite dem Umgebungsdruck ausgesetzt ist, derart, daß sich die Wand
(74) relativ zu dem Gehäuse (70) bewegt, um auf das Stützmedium (60) zu drücken, wenn
die Kammer (76) evakuiert wird.
16. Vorrichtung nach Anspruch 14, bei der die bewegliche Wand eine gasundurchlässige,
flexible Stirnwand (112') des Gehäuses (70') umfaßt.
17. Vorrichtung nach Anspruch 12, bei der die Einrichtungen zum Ausüben eines Druckes
auf das Stützmedium eine mit Druck beaufschlagbare Blase (120'') umfassen, die in
Kontakt mit dem Stützmedium (60'') in der Kammer (76'') angeordnet ist.
18. Vorrichtung nach Anspruch 12, bei der das feuerfeste, partikelförmige Stützmedium
losen Formsand umfaßt.
19. Vorrichtung nach Anspruch 12, bei der das Gehäuse (70) eine Vakuumkammer (76) aufweist,
wobei das lose feuerfeste, partikelförmige Stützmedium (60) in der Kammer (76) angeordnet
ist, um um den Mantel (30) herum zusammengepreßt zu werden, und wobei der unten liegende
Einlaß (90) für geschmolzenes Metall außerhalb der Vakuumkammer (76) angeordnet ist.
1. Procédé de moulage d'un métal fondu, comprenant les étapes suivantes exécutées successivement
:
(a) la formation d'un modèle consommable (10) d'un article à mouler, le modèle comprenant
un matériau fusible qui se dilate par chauffage,
(b) l'enrobage du modèle (10) d'une matière réfractaire particulaire (22) de moule
en plusieurs couches pour la formation d'une carapace réfractaire (30) autour du modèle,
(c) le chauffage du modèle enrobé (40) afin que le modèle (10) soit retiré de la carapace
(30) et laisse une cavité de moulage (36) dans la carapace,
(d) la disposition d'une matière particulaire réfractaire (60) de support autour de
la carapace mince (30), la cavité de moulage (36) communiquant avec une entrée de
métal fondu (18, 18a) placée à l'extérieur de la matière de support,
(e) l'évacuation de la cavité de moulage (36), et
(f) le moulage du métal fondu dans la cavité évacuée de moulage (36) alors que l'entrée
de métal fondu communique avec une source de métal fondu et la carapace (30) est supportée
dans la matière de support (60),
le procédé de moulage étant caractérisé en ce que
il comprend une opération de moulage contre la pesanteur d'un métal fondu aspiré
vers le haut dans la cavité évacuée de moulage (36) lorsque l'entrée de métal fondu
(18, 18a) communique avec une source de métal fondu placée au-dessous,
au cours de l'étape (b) de formation de la carapace, l'application des couches
de la matière du moule est réglée de manière qu'elle forme une mince carapace réfractaire
(30) ayant une épaisseur de paroi qui ne dépasse pas 3 mm environ (0,12 pouce), et
le procédé comporte en outre l'application d'une pression à la matière de support
(60) en même temps que la cavité de moulage (36) est évacuée afin que la matière de
support placée autour de la mince carapace (30) soit comprimée et supporte la carapace
lorsqu'elle encaisse les contraintes de moulage.
2. Procédé selon la revendication 1, dans lequel la mince carapace en couches (30) est
formée de manière qu'elle possède une perméabilité aux gaz supérieure au double de
celle d'une carapace analogue ayant deux fois son épaisseur de paroi.
3. Procédé selon la revendication 2, dans lequel la carapace (30) est formée de manière
qu'elle présente une perméabilité aux gaz au moins trois fois supérieure à celle que
présente la carapace analogue.
4. Procédé selon la revendication 1, dans lequel, au cours de l'étape (c), le modèle
enrobé (10) est traité à l'autoclave par de la vapeur d'eau afin que le modèle soit
retiré de la carapace mince (30) et laisse une cavité de moulage (36).
5. Procédé selon la revendication 1, dans lequel, au cours de l'étape (d), la carapace
(30) est supportée dans la matière de support (60) à l'intérieur d'un boîtier sous
vide (70), et un dispositif (74) de transmission de pression est déplacé par rapport
au boîtier sous vide (70) et à la matière de support (60) lorsque le boîtier est évacué
afin qu'il exerce la pression sur la matière de support.
6. Procédé selon la revendication 5, dans lequel le dispositif de transmission de pression
comporte une paroi mobile (74) du boîtier (70) destinée à exercer une pression sur
la matière de support (60).
7. Procédé selon la revendication 6, dans lequel la paroi (74) est soumise à un vide
relatif à sa face interne et à la pression ambiante à sa face externe.
8. Procédé selon la revendication 5, dans lequel le dispositif de transmission de pression
comporte une vessie (120'') placée au contact de la matière de support dans le boîtier
(70'') et mise sous pression afin qu'elle comprime la matière de support lorsque la
cavité de moulage (36'') est évacuée.
9. Procédé selon la revendication 1, dans lequel le modèle consommable (10) est formé
de cire.
10. Procédé selon la revendication 1, dans lequel le modèle consommable (10) est formé
d'urée.
11. Procédé selon la revendication 1, dans lequel l'étape (d) de disposition de la matière
de support (60) autour de la carapace comprend la disposition d'une matière particulaire
réfractaire de support autour de la carapace mince, contenue dans une chambre sous
vide, la cavité de moulage communiquant avec une entrée inférieure (18, 18a) de métal
fondu placée à l'extérieur de la chambre sous vide.
12. Appareil de moulage d'un métal fondu contre la pesanteur, comprenant :
(a) une matière particulaire réfractaire de support (60) disposée dans un boîtier
(70),
(b) une carapace réfractaire (30) ayant une cavité de moulage (36) délimitée par une
paroi de moule placée dans la matière de support (60),
(c) un dispositif (78, 80, 82) d'évacuation de la cavité de moulage (36), et
(d) une entrée (90) de métal fondu placée à l'extérieur de la matière de support (60)
afin que la cavité de moulage (36) et une source (102) de métal fondu communiquent,
lorsque la cavité de moulage (36) est évacuée, si bien que le métal fondu est chassé
dans la cavité évacuée de moulage (36),
l'appareil étant caractérisé en ce que la carapace a une épaisseur de paroi de
moule qui ne dépasse pas 3 mm (0,12 pouce), et
l'entrée de métal fondu est une entrée inférieure (90) destinée à faire communiquer
la cavité de moulage (36) avec une source (102) de métal fondu qui est placée au-dessous,
et un dispositif (74) est destiné à appliquer une pression à la matière de support
(60) pendant l'évacuation de la cavité de moulage (36), la pression étant telle que
la matière de support (60) est comprimée autour de la carapace (30) afin que celle-ci
soit supportée lorsqu'elle subit les contraintes de moulage.
13. Appareil selon la revendication 12, dans lequel l'entrée de métal fondu comprend un
tube (90) de remplissage dépassant de la carapace (30) à l'extérieur de la matière
de support (60).
14. Appareil selon la revendication 12, dans lequel le dispositif d'application d'une
pression à la matière de support (60) comporte une paroi mobile (74) du boîtier (70),
la paroi mobile (74) étant soumise à une pression différentielle lorsqu'une chambre
(76) du boîtier (70) est évacuée afin que la paroi (74) se déplace par rapport au
boîtier (70) et à la matière de support (60) et comprime la matière de support autour
de la carapace (30).
15. Appareil selon la revendication 14, dans lequel la paroi mobile comporte une paroi
(74) d'extrémité perméable aux gaz du boîtier, la paroi d'extrémité ayant une face
interne destinée à être au contact de la matière de support (60) et une cloche sous
vide (77) recouvrant sa face externe, la cloche sous vide (77) pouvant être évacuée
à l'extérieur afin que la chambre (76) soit évacuée par l'intermédiaire de la paroi
d'extrémité perméable aux gaz (74) et étant soumise à la pression ambiante à l'extérieur
de manière que la paroi (74) se déplace par rapport au boîtier (70) et exerce une
pression sur la matière de support (60) lorsque la chambre (76) est évacuée.
16. Appareil selon la revendication 14, dans lequel la paroi mobile comporte une paroi
souple d'extrémité (112') du boîtier (70') qui est imperméable aux gaz.
17. Appareil selon la revendication 12, dans lequel le dispositif d'application d'une
pression à la matière de support comporte une vessie (120'') qui peut être mise sous
pression et qui est placée au contact de la matière de support (60'') dans la chambre
(76'').
18. Appareil selon la revendication 12, dans lequel la matière particulaire réfractaire
de support comporte un sable fluide de fonderie.
19. Appareil selon la revendication 12, dans lequel le boîtier (70) a une chambre sous
vide (76),
la matière particulaire réfractaire fluide de support (60) est placée dans la chambre
(76) afin qu'elle soit comprimée autour de la carapace (30), et
l'entrée inférieure (90) de métal fondu est placée à l'extérieur de la chambre
sous vide (76).