Note: Descriptions are shown in the official language in which they were submitted.
633
BACKGROUND OF TEIE INVE~TIOI~
This invention relates to a new and improved mold
assembly and a method by which it is made and more
specifically to a segmented ceramic mold assembly-which may be
advantageously utilized in casting articles having portions o
different thicknesses.
When an article having relatively thick and thin portions
is to be cast, difficulty may be encountered due to
solidification of the relatively thin portion of the article
while the relatively thick portion of the article is still
molten. This can result in the formation of defects in the
thick portion of the article. These defects may be
detrimental to the operating characteristics of the article
and could result in premature failure of the article under
load. For ;example, turbine engines frequently include a
rotatable hub which is integrally cast with radially
projecting vanes or airfoils. The hub is relatively thick
while the vanes are relatively thin. When the hub and vanes
are to be integrally cast, difficulties may be encountered due
to solidification of the vanes ~hile the hub is still molten.
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The present invention provides an improved method of
making an improved mold assembly. The improved method could
be utilized to make molds for shaping many different objects.
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However, the method is advantageously utilized in making a mold assembly
which is utilized in the casting of a one piece turbine engine component
having portions of different thicknesses.
The mold assembly includes a plurality of interconnected sections.
The section of the mold assembly in which a relatively thin portion of an
article is to be cast has a lower rate of heat removal than the section of
the mold assembly in which a relatively thick portion of the article is to
be cast. As is well known, the heat removal rate varies as a function of
the thermal conductivity, specific head and density of the mold material.
According to one aspect of the invention there is provided a method
of making a mold assembly having portions of different thicknesses, said
method comprising the steps of providing a plurality of separate disposable
patterns each of which has a surface area with a configuration similar to a
portion of a surface area of a product, at least partially coating each of !
the disposable patterns with a wet covering of ceramic mold material, drying
~; the wet covering on the patterns, repeating the coating and drying steps
with a first one of the plurality of patterns a first number of times to
build up a relatively thick covering of ceramic mold ma~erial on the first
pattern, repeating the coating and drying steps with a second one of the
plurality of patterns a second number of times which is less than the first
number of times to build up a relatively thin covering of ceramic mold
material on the second pattern, separating the relatively thick covering of
ceramic mold material from the first pattern to provide a relatively thick
walled mold section, separating the relatively thin covering of ceramic mold
material from the second pattern to provide a relatively thin walled mold
section, and interconnecting the relatively thick and thin walled mold
sections to at least partially form the mold assembly.
According to another aspect of the invention there is provided
a method of making a mold assembly comprising the steps of providing a
plurality of separate disposable patterns each of which has a surface area
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with a configuration similar to a portion of a surface area of a cast
product, at least partially coating each of the disposable patterns with a
wet covering ceramic mold material, drying the wet coverings on the patterns,
repeating the coating and drying steps until coverings of ceramic mold
material of desired thicknesses have built up on the patterns, said step
of coating t}ie patterns includes at least partially covering a first
pattern with material having a first rate of heat removal, said step of
coating the patterns further includes at least partially covering a second
pattern with a material having a second rate of heat removal, the second
rate of heat removal being greater than the first rate of heat removal,
separating the covering from the first pattern to at least partially form
a f~irst mold section, separating the covering from the second pattern to
at least partially form a second moId section having a rate of heat removal
which is greater than the rate of heat removal of the first mold section,
and interconnecting the first and second mold sections to at least partially ~-
~: form a mold assembly having portions with different rates of heat removal.
- BRIEF DESCRIPTION OF THE DRAWINGS
~ The foregoing and other features of the present invention will
.i become more apparent upon a consideration of the following description taken
in connection with the accompanying drawings wherein: :
Figure 1 is an illustration of a cast turbojet engine fan frame;
-~ Figure 2 is an illustration of a mold assembly utilized to cast
the jet engine fan frame of Figure 1 and constructed in accordance with the
present invention;
Figure 3 is a radial sectional view further illustrating the
:~ configuration of various sections of the mold assembly of Figure 2;
Figure 4 is a fragmentary upwardly facing view of a hub portion of
the mold assembly of Figure 2 with some of the mold sections removed to
further illustrate the segmented construction of the mold assembly;
Figure 5 is an illustration of a pattern utilized in forming hub
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sections of the mold assembly of Figure 2;
Figure 6 is an illustration of a turbine engine component having
a relatively thick hub section and relatively thin vane sections;
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Fig. 7 is a fragmentary sectional view of a portion of a
¦ mold assembly having thick and thin walled mold sections; and
I Fig. 8 is a fragmentary sectional view of a portion of a
¦ mold assembly having sections with walls of different
¦ compositions.
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DESCRIPTION OF SPECIFIC PREFERRED
~vnooT~s~-~ O~ TUV-U~-O~
A fan frame or inlet duct 20 for a turbojet engine is
illustrated in Fig. 1. The jet engine fan frame 20 has an
annular central hub or wall 22 from which a plurality of
struts or vanes 24 extend radially outwardly to a relatively
large diameter annular outer ring or wall 26. When the fan
frame 20 is installed in a turbojet engine, the inner wall or
hub 22 supports one end of the compressor rotor. The struts
or vanes 24 direct air flow back to the compressor through the
space between the outer ring or wall 26 and hub. The hollow
struts 24 are also utilized to enclose conduits and other
parts (not shown) leading between the outside of the outer
ring 26 and the interior of the hub 22.
Since the outer ring 26 of the jet engine fan frame 20
has a relatively large diameter, that is a diameter in excess
of forty inches, and since relatively close dimensional
tolerances are required to fabricate a fan frame which will
function properly in a jet engine, relatively large fan frames
have previously been fabricated by joining a large number of
castings, sheet metal details and forgings to form a completed
assembly. Although only jet engine fan frame 20 has been
illustrated in Fig. 1, it should be understood that the
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present invention can advantageously be utilized in the ¦ -
forming of other turbine engine components. Among these other
turbine engine components are diffuser cases, nozzle rings,
vane assemblies and bearing supports.
The jet engine fan frame 20 is cast in one piece in a
segmented mold assembly 30 (see Fig. 2). The mold assembly 30
is constructed in the manner disclosed in United States Patent
No. 4,066,116 and includes a plurality of sprue or pour cups
32 which are disposed within a hub portion 34 of the mold
assembly. The hub portion 34 of the mold assembly 30 is
connected with an annular outer ring portion 36 of the mold
assembly by a plurality of radially extendlng strut portions
38 of the mold assembly.
As is perhaps best seen in Fig. 3, each of the pour cups
32 is connected in direct fluid communication with the hub
portion 3~ ~f the mold assembly 30 by gating 42. The hub
portion 34 of the mold assembly 30 is in turn connected in
fluid communication with the outer ring 36 of the mold
assembly through struts 38. Although the illustrated gating
42 onl~ connects the pour cup 32 with the hub portion 34 of
the mold assembly 30, additional gating and/or pour cups could
be provided in association with the outer ring portion 36 of
the mold assembly if desired. Upon a pouring of molten metal
into the pour cups 32 of the mold assembly 30, the metal flows
into an annular hub mold cavity 46 (Fig. 3), the radially
extending strut mold cavities 48 and into an annular outer
ring mold cavity 50. This results in an integrally cast jet
engine fan frame 20 having a one-piece construction.
633
The mold assemb]y 30 is formed of a plurality of mold
sections which are interconnected to define the various mold
cavities 46, 48 and 50. Although the ~et engine fan frame
mold assembly 30 is relatively large, by forming the mold
assembly 30 of a plurality of small mold sections, it is
possible to accurately form each of the mold sections. These
mold sections may then be placed in a jig or locating frame to
accurately position them relative to each other and are
cemented or otherwise interconnected to form a unitary
assembly.
The various mold sections are constructed in such a
manner that the surfaces which define the various mold
cavities can be readily inspected prior to construction of the
mold assembly 30. Of course, if any defects are noted during
the inspection they are either repaired or a properly formed
mold section is substituted for the defective mold section.
To this end, the hub portion 34 of the mold assembly 30
includes a circular array of hub panel mold sections 54 (see
Fig. 4) having major side surfaces 56 with a configuration
corresponding to the configuration of portions of an annular
inner side surface 58 (see Fig. 1) of the jet engine fan frame
hub 22. A second circular array of hub panel mold sections 58
are disposed radially outwardly of the hub mold panel sections
54 (see Fig. 4). The hub panel mold sections 58 have major
inner side surfaces 60 of a configuration corresponding to the
configuration of portions of the outside surface 64 (see Fig.
1) of the hub 22.
A plurality of top caps or end walls 68 extend between
the coaxial circular arrays of hub panel mold sections 54 and
633
58 to close off the top of the hub mold cavity 46. Similarly,
bottom caps or end walls 72 cooperate with the lower edge
portions of the hub panel mold sections 54 and 58 to close off
the bottom of the hub mold cavity 46 tsee Fig.s 3 and 4). The
mold sections 54 and 58 may be assembled in an inverted
position on a suitable jig or fixture so that the relatively
large diameter portion of the hub is disposed downwardly.
The outer ring portion 36 of the mold assembly 30 is
constructed in much the same manner as is the hub portion 34 h
of the mold assembly 30. Th,ls, the outer ring portion 36
includes a circular array of ring panel mold sections 76 (Fig.
2) having inner surfaces of a configuration corresponding to ~;
the configuration of portions of an annular inner side surface -
78 (Fig. 1) of the jet engine fan frame 20. A second circular
array of ring panel mold sections 82 (Fig. 2) is disposed
outwardly of and coaxial with the inner circular array of ring
panel mold sections 76. The mold sections 82 have inner or
mold surfaces which correspond to the configuration of
portions of the annular outer surface 84 (Fig. 1) of the outer
ring section 26 of the jet engine fan frame.
The upper and lower end portlons of the outer mold
sections 76 and 87 are interconnected by end caps or pane]s 88
and 90 (Fig. 3). The end caps 88 and 90 cooperate with the
outer ring panel mold sections 76 and 82 to close the outer
ring mold cavity 50 in the same manner as previously described
in connection with the hub mold end walls or caps 68 and 72.
The circular arrays of outer ring mold sections 76 and 82
circumscribe and are disposed in a coaxial relationship with
the circular arrays of hub panel mold sections 54 and 58.
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633
Both the hub portion 34 and outer ring portion 36 of the
mold assembly 30 are formed by separate mold sections so that
the surfaces which are utilized to form the molten metal in
either the annular hub mold cavity 46 or the annular outer
ring mold cavity 50 are exposed to view so that they can be ,
inspected. Of course, defective mold sections would be either
repaired or replaced. This results in high quality castings
which need little or no repair. Since the jet engine fan
frame 20 is integrally cast as one piece, the extensive
welding and brazing steps currently used to make large jet
engine fan frames are unnecessary.
The relatively large jet engine fan frame 20 is
integrally formed of a one-piece construction by a precision
investment casting or lost wax process. In this process the
wax patterns having configurations corresponding to the ~ :
configurations of the various mold sections are dipped in a
slurry of ceramic mold material. After the wax patterns have
been repetitively dipped and dried to form a covering of a
desired thickness over the wax pattern, the covering and
pattern are heated to a temperature sufficient to melt the wax
pattern so that the covering over the wax pattern is free of
the pattern. The mold could be dewaxed by many other methods
including using solvents or microwave energy. At least some
of the wet slurry coatings are wiped away from portions of the
wax pattern so that the various mold sections can be easily
separated when the wax pattern is melted. These mold sections
are then assembled in a suitable jig to form the mold assembly
30 0 ~ig. 2.
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,
A wax pattern (not shown) is utilized in forming of the
strut mold sections in the manner disclosed in United States
Patent No. 4,066,116. A wax pattern 174 (Fig 5) is utilized
to form the hub panel mold sections 54 and 58 (Fig. 3) and the
grating 42. Wax patterns of a configuration similar to the
wax pattern 174 (Fig. 5) but without the grating, are utilized
in the forming of the outer ring panel mold sections 76 and
82. It should be understood that the disposable patterns
could be formed of a material other than wax, for example, a
plastic pattern material such as polystyrene could be
utilized, if desired.
To form the hub panel mold sections 54 and 58, the wax
pattern 174 is repetitively dipped in a liquid slurry of
ceramic mold material. Although many different types of
slurry could be utilized, one illustrative slurry contains
fused silica, zircon, or other refractory materials in
combination with binders. Chemical binders such as ethyl
silicate, sodium silicate and colloidal silica can be
utilized. In addition, the slurry may contain suitable film
formers such as alginates to control viscosity and wetting
agents to control flow characteristics and pattern wetability. ~
In accordance with common practices, the initial slurry ;
coating applied to the pattern contains a very finely divided
refractory material to produce an accurate surface finish. A
typical slurry for a first coat may contain approximately 29
percent colloidal silica suspension in the form of a 20 to 30
percent concentrate. Fused si]ica of a particle size of 325
me6h o small6r in an amount of 71 percent can be employe2, ---
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633
together with less than one-tenth percent by weight of a
wetting agent. Generally, the specific gravity of the slurry
of ceramic mold material may be on the order of 1.75 to 1.80
and have a viscosity of 40 to 60 seconds when measured with a
Number 5 Zahn cup at 75 to 85F. After the application of
the initial coating, the surface is stuccoed with refractory
materials having particle sizes on the order of 60 to 200 mesh.
In accordance with well known procedures, each dip
coating is dried before subsequent dipping. The pattern is
repetitively dipped and dried enough times to build up a
covering of ceramic mold material of a desired thickness. In
one specific case the pattern was dipped fifteen times to
build up a covering of a thickness of approximately 0.400
inches in order to prevent mold bulge. After the dewaxing,
mold sections are fired at approximately 1900F. for one hour
to thoroughly cure the mold sections.
To provide the de-sired mold section configuration~ the
wax pattern 174 (see Fig. 5) includes a main wall or panel
section 176 having an arcuate configuration with an annular
extent of sixty degrees. The main wall section 176 includes a
radially inner major side surface 178 having a configuration
corresponding to the configuration of the radially inner
surface 58 (Fig. 1) of the air frame hub 22. A radially outer
major side surface 180 of the wall panel 176 has a
configuration corresponding to the configuration of the outer
surface 64 of the air frame hub 22. It should be noted that a
projection 184 is provided on the inner side of the wall 176
to form an opening to an associated strut section. Similarly,
a projection (not shown) is formed on the opposite side of the
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633
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wall 176 to form a root or base to which to connect the strut
mold sections.
Since each of the hub panel mold sections 54 and 58 are
connected with adjacent mold sections at flange joints,
pattern flange panels 188 and 190 (Fig. 10) are provided at
opposite ends of the main wall 176. The pattern flange panels
have inwardly facing side surface areas 192 and 194 which will
accurately form the flat flange surfaces of the hub panel mold
sections 54. Similarly, the flange panels 188 and 190 each
have a pair of facing side surface areas 198 (only one of
which is shown in Fig. 5) which accurately form the flat
flange surfaces on the outer hub panel mold section 58. The ,
flange panel 188 has a flat rectangular major outer side ,
surface 202 which is connected with the major side surface
areas 192 and 198 by a plurality of longitudinally extending
edge or minor side surfaces 204, 206, 208 and 210. Although
the configuration of only the flange panel 188 is fully ~ -
illustrated in Fig. 5, it should be understood that the flange
panel 190 is of the same configuration. It should be noted
that the major side surface 202 and the minor side surfaces
204, 206, 208 and 210 of the pattern flange panel 188 do not
correspnd to any surfaces on the hub panel mold sections 54
and 58.
Since the major outer side surfaces 202 of the pattern ;~-,
flange patterns 188 and 190 do not correspond to portions of
the hub mold sections, the ceramic coating on these outer side
panels must be separated from the,ceramic coatings on the wall
surfa :s 178 and 180 and ~he inner side surface areas 192, 194
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and 197 of the flange panels. In addition, the ceramic mold
material which was disposed over the inner major side surface
178 of the pattern wall 176 must be separated from the ceramic
mold material which was disposed over the outer major side
surface 180 of the mold wall 176.
The separating of the hardened ceramic mold material
overlying the major outer side surfaces 202 of the pattern
flange panels 188 and 190 from the hardened ceramic mold
material overlying the major side surfaces 178 and 180 of the
,.
panel wall 176 is greatly facilitated by wiping away the wet
¦i dip coating on the minor side surfaces of the flange panels
ll immediately after the pattern is dipped in the slurry of
¦¦ ceramic mold material. Similarly, the separating of the
hardened ceramic mold material overlying the inner and outer
major side surfaces 178 and 180 of the panel wall 176 is
l~ facilitated by wiping away the wet coating of ceramic mold
¦I material from upper and lower minor edge wall areas 214 and
¦l 126 extending between the upper and lower edges of the major
¦¦ side surfaces 178 and 180 of the wall panel 176.
¦ The manner in which the wiping away of the wet coating of
¦ ceramic mold material overlying the various minor side or edge
surfaces of the pattern 174 is performed is fully disclosed in
I United States Patent No. 4,066,116. After the pattern 174 has
¦ been dipped in a liquid slurry of ceramic mold material, the
pattern is manually supported above the liquid slurry tank by
a support frame. A metal blade is utilized to wipe away the
I! slurry coating overlying the edge surface 120 of the pattern
flange panel 188. Of course, the other minor surfaces 204,
206 an9 ~08 of the pattern Flange panel 188 are also wiped
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with the blade to remove the wet coating of ceramic mold
material overl~;ng the suxfaces. This separates the portion
of the wet coating of ceramic mold ~aterial overlying the
flange side surface 202 from the wet coating of ceramic mold
material overlying the remainder of the pattern 174. The wet
coating of ceramic material is then wiped from the minor sides
of the pattern flange panel 190. This separates the portion
of the coating of wet ceramic mold material overlying the
major side surface of the flange 190 from the wet coating of
ceramic mold material overlying the rest of the pattern 174.
The portions of the coating of wet ceramic mold material
overlyinq the major side surfaces 178 and 190 are separated
from each other. To this end, the wet coating of ceramic mold
material overlying the minor side edge surface 126 is wiped
away. Finally, the top edge surface 124 of the pattern 173 is
wiped with;the blade 218 to complete the removal of the wet
coating of ceramic mold material from the connect;ng surfaces
of the pattern 174.
It should be noted that the foregoing wiping steps
separated the wet coating of mold ceramic material overlying
the pattern 174 into a plurality of discrete segments each of
whcih is separated from an adjacent segment by a wiped area.
In the illustrated embodiment of the invention two of the
segments of wet dip coating correspond to two mold sections.
Thus, the segment of wet dip coating overlying the inner major
side surface 178 of the pattern corresponds to a hub mold
section 54 and the segment of the wet dip coating overlying
the ma or outer side sur~ac~ lBO of the pattern wall 176
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corresponds to the hub mold section 58. The segments of wet
dip coating overlying the major outer side surfaces of the
pattern flange panels 188 and 190 do not correspond to any of
the mold sections.
As the pattern 374 is repetitively dipped, each wet
coating is wiped in the manner previously explained and then
dried. This results in the formation of a multi-layered
covering of ceramic mold material on the pattern. This
covering of ceramic mold material is sharply discontinuous at
the areas overlying the wiped surfaces of the pattern. Thus
the wiped minor flange surface 204 of the pattern flangè panel
188, a covering 218 of ceramic mold material overlying the
flange panel side surface 202 is separated from a covering 220
overlying the inner side surface 198 of the inner flange panel
1ange 188 and the major side surface 178 of the pattern wall
176. When ;the wax pattern is disposed of by melting, the
dried covering 128 of ceramic mold material overlying the
pattern flange panel surface 202 is separated from the dried
covering 220 of ceramic mold material overlying the pattern
flange panel surface 198 and side wall surface 178.
Similarly, a covering 224 of dried ceramic mold material
overlying the pattern flange surface lg8 and the outer pattern
wall surface 180 is separated from the covering 128 overlying
the major outer surface 202 of the pattern flange panel. r
:' l
1 A turbine engine component 280 having a relatively thick
. ~ hub po ion 282 from which relatively thin vanes or airfoils
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284 extend radially is illustrated in Fig. 6. The hub 282 and
vanes 284 are integrally cast as one piece. During the
casting process, the relatively thin vanes 284 tend to
solidify before the relativeIy thick hub 282 solidifies. If
the hub 282 is allowed to remain molten after the vanes have
solidified, defects may be formed in the hub. Of course, any
defects which are formed in the hub 282 are detrimental to its
strength. Although many different types of defects could
develop with different metals, microporosity and inclusions
are the most common defects to be eliminated. In addition,
controlling the heat removal rate enables grain structure and
size to be controlled.
Although the turbine engine component 280 has been
illustrated in Fig. 6 as having vanes 284 which are not -
associated with an outer ring or shroud similar to the outer
ring 26 of the turbine engine component 20 of Fig. 1, it is
contemplated that such an outer ring could be associated with
the turbine engine component 280. If this were done, the ~ ,
outer ring could solidify after the vanes 284 solidify and
before the hub 282 solidifies with the formation of defects in
both the hub and outer ring.
In accordance with a feature of the present invention,
the turbine engine component 280 is cast in a mold assembly
288, a portion of which is shown in Fig. 7. The mold assembly
288 is formed of a plurality sections which are interconnected
to define relatively small cavities in which the vanes 284 are
cast and a relatively large cavity in which the hub 282 is
cast. The various sections are interconnected in the same
manner as previously explained in connection with the mold
assembly 30.
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,
¦ The mold assembly 288 includes sections having different
l heat removal rates. Thus, a mold section 290 in which a vane
or airfoil 284 is cast has a relatively low rate of heat
il removal whlle a mold section 294 in which the hub 282 is cast
¦I has a high rate of heat removal~ The different rates of heat
li removal result in a promoting of solidification of the
¦¦ relatively thick hub 282 and retarding of solidification of
the relatively thin vanes 284. This tends to minimize any
tendency for defects to form in the hub as it solidifies.
It is contemplated that the different heat removal rates -
for the mold sections 290 and 294 could be obtained in several
different ways. In the embodiment of the invention
illustrated in Fig. 7, the vane mold section 290 has a
relatively thick wall to retard removal of heat from the
relatively thin vanes or airfoils 284. The hub mold sectio-n
~;i' 294 has a-relatively thin wall to enable heat to be readily
¦ removaved from the relatively thick hub 282.
¦ The different mold wall thicknesses are obtained by
¦ dipping the associated patterns a different number of times in
¦ a slurry of liquid ceramic mold material. Thus, a wax or
plastic pattern having a conflguration corresponding the shape
of a single vane 284 is dipped in a liquid ceramic mold
material a relatively large number of times, for example
twelve times, to form a relatively thick build-up of ceramic
mold material over the pattern. Each time the vane pattern is
dipped, it is wiped in the manner previously explained to
remove the wet ceramic mold material from an area where a -
joint is to be formed between the mold sections 290 and 294.
The relatively thick wall of the vane mold section 290 has a
relatively low rate of heat removal and tends to maintain the
vane molten while the hub is solidifying.
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,
To promote the solidification of the hub, the mold
section 294 has a relatively thin wall. The relatively thin
wa]led mold section 294 was obtained by dipping a hub pattern
a relatively small number of times in liquid ceramic mold
material. For example, the hub pattern associated with the
mold section 294 was dipped only six times to provide a
relatively thin build-up of ceramic mold material. The thin
walled hub mold section 294 and a plurality of thick walled
vane mold sections 290 are interconnected in the manner -
previously explained to form a mold assembly in which the
turbine engine component 280 is cast.
The thin walled hub mold section is not as effective to
insulate the hub as the relatively thick walled vane mold
sections 290. Therefore, the heat transfer rate from the hub
is greater than the heat transfer rate from the vanes to
promote solidification of the hub contemporaneously with
solidification of the vanes. The transfer of heat from the
hub can be further promoted by investing the mold 288 in a
container with steel shot adjacent to the hub mold section 294
to provide a heat sink.
It is also contemplated that the mold sections could be
provided with different heat removal rates by forming the mold
sections of different materials. Thus, the pattern for a vane
mold section 298 (Fig. 8) is dipped in a slurry of colloidal
silica in which Zircon is suspended. The resulting wet
covering of ceramic mold material is coated with a stuccoing
of fused silica having a particle size on the order of 60 to
20 mesh This wet coating is then dried. After repetitive
dipping, stuccoing and drying, the resulting covering is
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63~ ,
separated from the pattern to form a ceramic mold section in
the manner previously explained.
A hub mold section 304 is formed by dipping a pattern
having a configuration corresponding the configuration of the
!i
hub 282 in a slurry o~ colloidal si]ica having the same
¦, composition as the slurry in which the vane pattern was
¦1 dipped. However, the wet covering of ceramic mold material on
; I the hub pattern is stuccoed with zircon. The wet covering of
I silica stuccoed with zircon is then dried. After repetitive
¦ dipping, stuccoing and drying, the resulting covering is ~
¦ separated from the pattern. Due to the zircon stuccoing, the
resulting hub mold section has a heat removal rate which is
I greater than the heat removal rate of the vane mold section
298 formed by coating a wet slurry covering of silica with a
stuccoing of fused silica. Of course, other stuccoing
materials having a rela,ively high heat removal rate could be
utilized if desired.
~1 The two mold sections 298 and 304 are interconnected at
joints in the matter previously explained in connection with
the mold assembly 30. This results in a mold assembly 306
having a hub mold section 304 with a relatively high heat
removal rate and a vane mold section 298 with a relatively low
heat removal rate. Although the mold sections 298 and 304
have the same thickness, it is contemplated that they could be
formed with different thicknesses by coating the associated
¦ patterns different numbers of times with ceramic slurry and
stucco.
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633
It is contemplated that it may be desirable under certain
circumstances to form both of the mold sections of completely
different materials. This could be done by repetitively
dipping the hub pattern in a s]urry of ceramic mold material
having a high heat removal rate. The vane pattern would be
repetitively dipped in a slurry of a different ceramic mold
material having a low heat removal rate. For example, a
pattern associated with a thick portion of an article could be
dipped in a slurry having a zircon filler while the other
pattern is dipped in a slurry having a fused silica filler.
Another way of controlling the heat removal rate of the ;-
mold sections is to form the mold sections with different ~
porosities. The vane mold section wou3d be made relatively ~r
porous to retard heat removal~ The hub mold section would be
relatively dense to promote heat removal.
In vi~W of the foregoing, it is apparent that the present
invention provides an improved method of making a mold
assembly having portions with different heat removal rates.
The mold assembly includes a plurality of interconnected
sections. The sections of the mold assembly in which
relatively thin portions of the article are to be cast-have a.
lower rate of heat removal than the sections of the mold
assembly in which relatively thick portions of the articles
are to be cast. Of course if desired, the mold could be
constructed to have a high heat removal rate from the thin
portion of the casting and a low heat removal rate from the
thick portion of the casting.
In one embodiment of the invention the different heat
removal rates are obtained by forming the mold assembly 288
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633
with walls of different thicknesses. A relatively thin walled
section 294 is utilized to define a mold in which the thick
hub portion of the article is cast. Thick walled mold
sections 290 are utilized to define the mold cavities in which
the thin vane portions of the article are cast. In another
embodiment of the invention the composition of the mold
sections are different to provide different heat removal
rates. The mold wall section 304 associated with the
relatively thick hub portion of a casting is formed of a
substance having a relatively high heat removal rate to
promote solidification of the hub portion of the casting. The
vane mold sections 298 are formea of a material having a
relatively low heat removal rate to retard solidification of
the vanes. .
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