U.S. patent application number 13/886449 was filed with the patent office on 2013-12-19 for investment casting utilizing flexible wax pattern tool for supporting a ceramic core along its length during wax injection.
The applicant listed for this patent is Iain Alasdair Fraser, Benjamin E. Heneveld, Allister Williams James, GARY B. MERRILL, Kevin C. Sheehan. Invention is credited to Iain Alasdair Fraser, Benjamin E. Heneveld, Allister Williams James, GARY B. MERRILL, Kevin C. Sheehan.
Application Number | 20130333855 13/886449 |
Document ID | / |
Family ID | 49754824 |
Filed Date | 2013-12-19 |
United States Patent
Application |
20130333855 |
Kind Code |
A1 |
MERRILL; GARY B. ; et
al. |
December 19, 2013 |
INVESTMENT CASTING UTILIZING FLEXIBLE WAX PATTERN TOOL FOR
SUPPORTING A CERAMIC CORE ALONG ITS LENGTH DURING WAX INJECTION
Abstract
An investment casting process wherein the wax pattern tool (42)
is flexible to provide compliant support for an enclosed ceramic
core (10) and to facilitate removal of the tool from the cast wax
pattern (52) even when the cast shape would otherwise require
multiple pull planes. Positioning pins (106) may extend from the
flexible tool to make compliant contact against the core during the
wax injection step. The pins may cooperate with a pedestal (128)
formed on the core to support the core along multiple axes during
wax injection, thereby allowing a higher wax injection pressure
without damage to the core.
Inventors: |
MERRILL; GARY B.; (Orlando,
FL) ; James; Allister Williams; (Chuluota, FL)
; Sheehan; Kevin C.; (Orlando, FL) ; Heneveld;
Benjamin E.; (Newmarket, NH) ; Fraser; Iain
Alasdair; (Ruckersville, VA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
MERRILL; GARY B.
James; Allister Williams
Sheehan; Kevin C.
Heneveld; Benjamin E.
Fraser; Iain Alasdair |
Orlando
Chuluota
Orlando
Newmarket
Ruckersville |
FL
FL
FL
NH
VA |
US
US
US
US
US |
|
|
Family ID: |
49754824 |
Appl. No.: |
13/886449 |
Filed: |
May 3, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12961740 |
Dec 7, 2010 |
|
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|
13886449 |
|
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Current U.S.
Class: |
164/45 ;
164/159 |
Current CPC
Class: |
B22C 9/02 20130101; B22C
7/02 20130101; B22C 21/14 20130101; B22C 9/24 20130101; B22C 9/04
20130101; B22C 9/103 20130101 |
Class at
Publication: |
164/45 ;
164/159 |
International
Class: |
B22C 7/02 20060101
B22C007/02 |
Claims
1. A method of producing a wax pattern for an investment casting
process, the method comprising: positioning a ceramic core within a
wax pattern mold; supporting the ceramic core from a flexible
surface of the mold; injecting melted wax into the mold around the
ceramic core; and removing the mold after the wax has hardened to
reveal the ceramic core encased inside a wax pattern.
2. The method of claim 1, further comprising injecting the melted
wax at a pressure of greater than 175 psig.
3. The method of claim 1, further comprising supporting the ceramic
core at a plurality of locations along its length during the step
of injecting melted wax by locating a respective plurality of pins
to extend from the flexible surface of the mold to make proximate
contact with the core along a respective plurality of non-parallel
axes.
4. The method of claim 1, further comprising supporting the ceramic
core at a location along its length during the step of injecting
melted wax by locating a pin to extend from the flexible surface of
the mold to make proximate contact with the core.
5. The method of claim 4, further comprising aligning the pin to
make proximate contact with a pedestal formed on the core to
provide support for the core along a plurality of axes.
6. The method of claim 5, further comprising forming the pedestal
to have an opening defined by sloping walls for receiving the
pin.
7. The method of claim 4, further comprising: forming a recess in
the flexible surface of the mold; disposing a supporting element
within the recess; and installing the pin through an opening in the
supporting element to extend toward the core.
8. The method of claim 7, further comprising forming the recess and
the supporting element to have cooperatively tapered sides.
9. The method of claim 4, further comprising: forming a recess in
the flexible surface of the mold; supporting the pin from a
fugitive material disposed within the recess; and removing the
fugitive material after the step of removing the mold.
10. The method of claim 1, further comprising inserting a precision
ceramic insert into a recess formed in the surface of the mold
prior to the wax injection step, and removing the mold after the
wax has hardened to reveal the wax pattern with the ceramic insert
retained on a surface of the hardened wax.
11. A tool for producing a wax pattern for an investment casting
process, the tool comprising: a tool body comprising a flexible
inner surface defining a desired outer surface geometry of the wax
pattern; a core disposed within the tool body, the core forming
part of the wax pattern after a wax injection step and defining a
wax injection volume between the core and the flexible inner
surface; and a metal pin supported by the flexible inner surface
and extending through the wax injection volume to a position
proximate the core effective for supporting the core during a wax
injection step.
12. The tool of claim 11, further comprising: a recess in the
flexible inner surface of the tool body; a supporting element
disposed within the recess; and an opening formed in the supporting
element for receiving an end of the pin.
13. The tool of claim 11, wherein the supporting element is formed
to have tapered sides shaped to cooperate with tapered sides of the
recess.
14. The tool of claim 11, further comprising: a plurality of metal
pins supported by the flexible inner surface and extending through
the wax injection volume to a plurality of positions proximate the
core effective for supporting the core along a respective plurality
of axes during the wax injection step.
15. The tool of claim 14, wherein at least two of the axes are not
parallel.
16. The tool of claim 11, further comprising a pedestal formed on
the core proximate the pin to provide mechanical interference with
the pin along a plurality of axes.
17. The tool of claim 16, further comprising an opening formed by
sloping walls of the pedestal for receiving the pin.
18. The tool of claim 16, wherein the pedestal comprises a
protruding undercut for creating mechanical interference with a
head of the pin.
19. The tool of claim 12, wherein the supporting element is formed
of a fugitive material.
20. The tool of claim 11, further comprising a precision ceramic
insert disposed on the tool body inner surface.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of co-pending
application Ser. No. 12/961,740 filed 7 Dec. 2010 (attorney docket
2010P13199US), which in turn claimed benefit of the 8 Dec. 2009
filing date of U.S. provisional application No. 61/267,519
(attorney docket 2009P22785US).
FIELD OF THE INVENTION
[0002] This invention relates generally to the field of investment
casting, and more particularly, to using a flexible tool or mold to
form a wax pattern as part of an investment casting process.
BACKGROUND OF THE INVENTION
[0003] Investment casting is one of the oldest known metal-forming
processes, dating back thousands of years to when it was first used
to produce detailed artwork from metals such as copper, bronze and
gold. Industrial investment castings became more common in the
1940's when World War II increased the demand for precisely
dimensioned parts formed of specialized metal alloys. Today,
investment casting is commonly used in the aerospace and power
industries to produce gas turbine components such as blades or
vanes having complex airfoil shapes and internal cooling passage
geometries.
[0004] The production of an investment cast gas turbine blade or
vane involves producing a ceramic casting vessel having an outer
ceramic shell with an inside surface corresponding to the airfoil
shape, and one or more ceramic cores positioned within the outer
ceramic shell corresponding to interior cooling passages to be
formed within the airfoil. Molten alloy is introduced into the
ceramic casting vessel and is then allowed to cool and to harden.
The outer ceramic shell and ceramic core(s) are then removed by
mechanical or chemical means to reveal the cast blade or vane
having the external airfoil shape and hollow interior cooling
passages in the shape of the ceramic core(s).
[0005] A ceramic core for injection casting is manufactured by
first precision machining the desired core shape into mating core
mold halves formed of high strength hardened machine steel, then
joining the mold halves to define an injection volume corresponding
to the desired core shape, and vacuum injecting a ceramic molding
material into the injection volume. The molding material is a
mixture of ceramic powder and binder material. Once the ceramic
molding material has hardened to a green state, the mold halves are
separated to release the green state ceramic core. The fragile
green state core is then thermally processed to remove the binder
and to sinter the ceramic powder together to create a material that
can withstand the temperature requirements necessary to survive the
casting of the molten alloy. The complete ceramic casting vessel is
formed by positioning the ceramic core within the two joined halves
of another precision machined hardened steel mold (referred to as
the wax pattern mold or wax pattern tool) which defines an
injection volume that corresponds to the desired airfoil shape, and
then injecting melted wax into the wax pattern mold around the
ceramic core. Once the wax has hardened, the mold halves are
separated and removed to reveal the ceramic core encased inside a
wax pattern, with the wax pattern now corresponding to the airfoil
shape. The outer surface of the wax pattern is then coated with a
ceramic mold material, such as by a dipping process, to form the
ceramic shell around the core/wax pattern. Upon sintering of the
shell and consequential removal of the wax, the completed ceramic
casting vessel is available to receive molten alloy in the
investment casting process, as described above.
[0006] It is further known to insert positioning wires or pins into
the wax pattern prior to coating the wax pattern with ceramic mold
material. The positioning wires are inserted through the wax until
they make only light contact with the encased ceramic core so that
further insertion of the wire is terminated prior to it causing
damage to the fragile ceramic core material. A portion of the wire
remains extending beyond the wax surface and is subsequently
encased within the surrounding ceramic mold material. The
positioning wires serve to provide mechanical support to the core
once the wax is removed and during the subsequent molten metal
injection step. The wire material, typically platinum, will melt
after the molten metal is injected into the completed ceramic
casting mold and becomes integrated into the final cast
product.
[0007] The known investment casting process is expensive and time
consuming, with the development of a new blade or vane design
typically taking many months and hundreds of thousands of dollars
to complete. Furthermore, design choices are restricted by process
limitations in the production of ceramic cores and wax patterns.
The metals forming industry has recognized these limitations and
has developed at least some incremental improvements, such as the
improved process for casting airfoil trailing edge cooling channels
described in U.S. Pat. No. 7,438,527. While incremental
improvements have been presented in the field of investment casting
technology, the present inventors have recognized that the industry
is faced with fundamental limitations that will significantly
inhibit component designs for planned advances in many fields, for
example in the next generation of gas turbine engines, where firing
temperatures continue to be increased in order to improve the
efficiency of combustion and gas turbine hot gas path component
sizes continue to increase as power levels are raised.
SUMMARY OF THE INVENTION
[0008] The present invention is part of an entirely new regiment
for investment casting, and it focuses on the use of a flexible
tool when casting a wax pattern around a ceramic core. In
particular, the ceramic core is supported from the flexible tool at
points along its length during the wax injection step. The present
invention allows higher pressures to be used for wax injection
while reducing the incidence of damage or movement of the core
during the wax injection step. The overall investment casting
process is described herein so that the reader may appreciate how
the present invention fits within and contributes to the new
regiment.
[0009] A flexible wax pattern mold may be formed as a hybrid tool
having a flexible insert within a coffin mold. The flexible insert
facilitates the removal of the wax pattern tool from the cast wax
pattern by deforming the flexible insert around cast features that
would otherwise require multiple pull planes for hard tooling. The
flexible insert may be cast from a master tool that is machined
from a relatively low cost, low hardness material such as aluminum
or mild steel.
[0010] Some desired surface topographies are so fine that they may
not survive in the surface of the wax during subsequent handling
and ceramic shell forming steps. For such embodiments, a ceramic
insert may be used in conjunction with the flexible wax pattern
mold. The ceramic insert may be formed to include a desired
intricate surface topography. The ceramic insert is set into the
flexible wax pattern mold and forms part of the surface which
defines the wax injection volume. After wax injection and
solidification, the ceramic insert remains adhered to the wax
pattern when the flexible wax pattern mold is removed.
Subsequently, the ceramic shell is formed around the wax pattern
and its adhered insert, such as by a dipping process as described
above, and the insert becomes an integral part of the ceramic shell
upon firing.
[0011] A ceramic insert placed into the flexible wax pattern mold
may also be used to define surface-opening passages in the
subsequently cast metal part, such as trailing edge cooling holes
for a gas turbine blade. In this embodiment, the ceramic insert
would include projections corresponding to a desired shape of the
cooling holes. The projections would extend to make contact with
the ceramic core, thereby defining cooling channels in the
subsequently cast part that extend from a hollow interior portion
of the blade (defined by the ceramic core) to the blade surface
(defined by the inside surface of the ceramic shell). The distal
end of the projection may be formed with a feature that mates with
a cooperating feature formed on the ceramic core. Mechanical
contact between the core and the ceramic insert projection serves
to precisely locate the ceramic core within the flexible wax
pattern mold and also to mechanically support the ceramic core
during the subsequent wax and metal injection steps.
[0012] The flexible insert of the wax pattern mold may be formed to
include alignment features that allow the insert to be precisely
located relative to the surrounding coffin mold, which in turn, can
precisely locate the insert and any feature formed on the insert
relative to the enclosed ceramic core for the wax injection
step.
[0013] The molding material used to form the flexible mold or
flexible mold insert may be infused with or cast around a material
or device which allows the flexible insert to react in a desired
way; broadly described herein as the flexible insert containing a
reactive element. The reactive element may be a filler material
which imparts a desired characteristic to the subsequently cured
material. For example, if magnetic particles are used as filler,
the cured flexible insert will be responsive to magnetic energy.
This characteristic may be useful for securing the flexible insert
within the surrounding coffin mold when the coffin mold is formed
to include permanent or electromagnets. If thermally conductive or
thermally insulating materials are used as filler, heat transfer
through the flexible insert may be more conveniently controlled
during its use.
[0014] Another type of reactive element that may be embedded within
the flexible mold or insert when it is formed is an active device.
Such active devices may include a temperature sensor, a pressure
sensor, a mechanical vibrator, a heating or cooling device, or
other device that may be useful when the flexible insert is used
during a subsequent wax injection process.
[0015] Positioning pins (wires) may be used with a flexible wax
pattern mold to mechanically support the enclosed ceramic core
during metal casting, and importantly, these may be positioned
against the ceramic core prior to the wax injection step.
Specialized pin supporting elements are located into recesses in
the surface of the flexible insert, thereby precisely positioning
the pins relative to the ceramic core prior to wax injection. This
allows the pins to support the core during wax injection and it
also allows the pins to be located with more precision than in the
prior art process which required the pins to be inserted through
the already-cast wax pattern. As a result, damage to the fragile
ceramic core is reduced and process yield is increased. The ceramic
core may be formed with a surface feature specifically dimensioned
and positioned to cooperate with the end of the pin in order to
allow the pin to provide support to the core along two axes.
[0016] An enabling technology which is exploited in the present
invention is described in U.S. Pat. Nos. 7,141,812 and 7,410,606
and 7,411,204, all assigned to Mikro Systems, Inc. of
Charlottesville, Va., and incorporated by reference herein. This
technology is commonly referred to as Tomo Lithographic Molding
Technology (hereinafter referred to as the "Tomo process"), and it
involves the use of a metallic foil stack lamination mold to
produce a flexible derived mold, which in turn is then used to cast
a component part. The component design is first embodied in a
digital model and is then digitally sliced, and a metal foil is
formed corresponding to each slice using photolithography or other
precision material removal process. The inherent precision of the
two-dimensional material removal process in combination with the
designer's ability to control the thickness of the various slices
in the third dimension provides a degree of three-dimensional
manufacturing tolerance precision that was not previously available
using standard mold machining processes. The foils are stacked
together to form a lamination mold for receiving suitable flexible
molding material. The term "flexible" is used herein to refer to a
material such as a room temperature vulcanizing (RTV) silicon
rubber or other material which can be used to form a "flexible
mold" which is not rigid like prior art metal molds, but that
allows the mold to be bent and stretched to a degree in order to
facilitate the removal of the mold from a structure cast therein.
Furthermore, the terms "flexible mold" and "flexible tool" may be
used herein to include a self-standing flexible structure as well
as a flexible liner or insert contained within a rigid coffin mold.
A component is then cast directly into the flexible mold. The
flexibility of the mold material enables the casting of component
features having protruding undercuts and reverse cross-section
tapers due to the ability of the flexible mold material to deform
around the feature as the cast part is pulled out of the mold.
[0017] Collectively, these improvements define a new regiment for
investment casting which overcomes many of the limitations of the
prior art, particularly limitations in the wax pattern portion of
the investment casting process, as more fully described below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The invention is explained in detail in the following
description in view of the drawings that show:
[0019] FIG. 1 is a prior art ceramic core.
[0020] FIGS. 2A-2B illustrate steps for manufacturing wax pattern
tooling for an investment casting process.
[0021] FIG. 3 illustrates a spacer located between a ceramic core
and a flexible wax pattern mold for positioning the core and
supporting the core during wax injection.
[0022] FIGS. 4A-4H illustrate steps of an investment casting
process wherein an engineered surface topography is cast directly
into the metal part surface.
[0023] FIG. 5 is a first wax pattern surface generated from
Tomo-process flexible tooling.
[0024] FIG. 6 is a second wax pattern surface generated from
Tomo-process flexible tooling.
[0025] FIG. 7 is a wax pattern surface with a protruding surface
pattern.
[0026] FIGS. 8A-8C show wax surfaces derived from a single master
tool subjected to progressive grit blasting.
[0027] FIGS. 9A-9C illustrate steps for producing an engineered
surface in an investment cast part.
[0028] FIGS. 10A-10B illustrate a wax pattern tool insert used to
define surface opening passageways in a cast metal part.
[0029] FIG. 11 is a partial cross-sectional view of a flexible wax
injection mold insert containing reactive (magnetic) particles and
magnets affixed to the coffin mold.
[0030] FIG. 12 is a partial cross-sectional view of a flexible wax
injection mold insert positioned in a coffin mold wherein the liner
encapsulates an active device.
[0031] FIGS. 13A-13E illustrate the use of a core positioning wire
with a flexible mold insert whereby the wire is positioned against
the core prior to the wax injection step.
[0032] FIG. 14 illustrates an insert having a comb design.
[0033] FIGS. 15 and 16 illustrate the use of a core positioning
wire which cooperates with a pedestal formed on the ceramic core to
support the core along multiple axes during wax injection.
[0034] FIG. 17 illustrates the use of a fugitive supporting element
for a core positioning wire disposed at a shallow angle relative to
a tool pull plane.
DETAILED DESCRIPTION OF THE INVENTION
[0035] As part of an investment casting process such as may be used
for casting a gas turbine blade or other component with complex
internal cooling passages, a ceramic core is first produced which
will define the shape of the internal cooling passages. FIG. 1 is
an illustration of one such ceramic core 10 which may be formed by
any known process.
[0036] Once the ceramic core is produced, the next step in the
investment casting process is to use the core as part of wax
pattern tool for casting wax around the core to define the eventual
outer surface shape of the cast blade or other cast part. Prior art
wax pattern tooling design is especially complicated and expensive
when multiple pull planes are required for removal of the tooling
from the wax pattern or cast part due to the geometry of the part.
The present invention provides a novel approach to wax pattern
tooling which reduces the tool manufacturing time and cost to a
small fraction of that required for traditional wax pattern
tooling, and further, provides improved capabilities that result in
greater component design flexibility and higher casting yields.
Simple, low cost aluminum or soft steel (or other easily machined
material, collectively referred to as a soft metal) master tooling
is used in lieu of the expensive machine tool steel tooling of the
prior art. A derived flexible wax pattern mold (tool) is then
produced from the master mold using a low pressure injection
process. FIG. 2A illustrates a section of a master tool 12 formed
of soft metal such as machined aluminum receiving flexible mold
material 14 to produce one side 16a of a flexible wax pattern tool
16, with the other side 16b of the flexible wax pattern tool 16
being produced in a similar manner. The flexible wax pattern tool
may be formed completely of the flexible mold material, or it may
take a hybrid form of a flexible mold insert (or liner) which is
used in conjunction with a solid coffin mold, which is illustrated
and discussed below. The two assembled sides of the flexible wax
pattern tool are shown in FIG. 2B, which illustrates the placement
of a ceramic core into the injection cavity 18 defined between the
flexible mold sides.
[0037] The master tool may be formed to receive one or more
precision inserts 20, which in the embodiment of FIGS. 2A-B are
used to form positioning features 22, such as in the shape of
flexible positioning pins, formed to be integral with the flexible
inner surface 24 of the mold, which abut against the core. The
precision inserts of various shapes may be used in any area of the
master tool if needed to define enhanced areas of high definition
detail, including for example, engineered surface roughness which
facilitates the adhesion of a coating to be applied to the
later-cast metal part, as described more fully below. The inserts
may be formed using a Tomo process, stereo lithography, direct
metal fabrication or other high definition process. The hybrid
surface 26 of the master tooling (machined aluminum surface and
precision insert surface) is then replicated in the flexible inner
surface of the wax injection mold which duplicates the detail of
the master tool.
[0038] The positioning features illustrated in FIG. 2B extend from
the body of the flexible mold with high mechanical hysteresis to
make gentle contact with the ceramic core, thereby ensuring the
proper position of the core within the flexible wax pattern mold.
The positioning features provide a degree of compliance at the
core/mold interface 28 while providing mechanical support to the
core during wax injection. Prior art tooling is known to
incorporate metal pins to make hard contact with the core, but such
hard contact often causes damage to the relatively fragile ceramic
core during mold closure. The flexible features described herein
provide a degree of forgiveness, which is translated into a higher
yield of acceptable parts due to reduced chance of damage to the
core. The degree of flexibility of the positioning features can
vary, but they may be more flexible than the core surface which
they contact so that they can be deformed by the core without
causing damage to the ceramic core material. In contrast, the prior
art positioning pins are more rigid than the core material, and
they cannot be deformed by the core without causing damage to the
ceramic core material. Advantageously, the positioning features
need not be located in parallel pull planes due to the flexible
nature of the mold material, which allows them to be bent to
facilitate removal.
[0039] In another embodiment illustrated in FIG. 3, a flexible pin
or spacer 30 that is not integral to the flexible mold may be
located between the core and the flexible mold in order to position
the core and to provide mechanical support to the core. Such a
non-integral spacer may be formed of foam or wax or any material
that can engage the ceramic core without causing damage to the
core. The spacer may be held in position with an adhesive 32 and/or
it may be inserted into an opening 34 formed in the flexible mold.
The spacer may be designed to burn out during shell hardening after
wax injection, or it may be pulled out of the wax pattern before
shelling. Alternatively, the spacer may be formed of a ceramic
casting material, and it may remain entrapped by the wax pattern
and then be coated and become integral with the subsequently
applied ceramic shell material. Upon molten metal alloy casting,
the entrapped ceramic spacer will function to define a
surface-opening passageway in the cast metal part. In any of these
options, the flexible inner surface of the flexible mold as well as
the flexibility of the spacer itself serve to provide a degree of
compliant support to the core during the wax injection step.
[0040] The above described regiment for producing wax pattern
tooling compares favorably with known prior art processes, as
summarized in the following Table 1.
TABLE-US-00001 TABLE 1 Prior Art Invention Characteristic
Characteristic Prior Art Capability Invention Capability Hard
Precision Soft Precision Tooling Tooling Linear extraction
Curvilinear extraction only. capability. Single cross section
Multiple cross section pull plane. pull planes. Hard pins locate
core Flexible mold within hard tool. extenders locate core within
flexible mold. Hard tool to core Soft tool to core Limited core to
tool Able to locate less interface. interface. interface provides
robust areas of core in lower certainty of soft tool interface.
core location. More control over core location. Inflexible tool
set, Low cost modular high cost to modify. modifications enabled.
Rigid mold cavity, Flexible mold cavity, precision linear tool
non-linear mold separation. separation.
[0041] FIGS. 4A-4H illustrate steps of an investment casting
process wherein an engineered surface topography 36 is cast
directly into the metal part surface 38. In FIG. 4A the two halves
of a coffin mold (die) 40 are shown, each containing a flexible
mold insert (liner) 42a, 42b having an exposed surface containing
the desired surface topography. The flexible inserts may be
produced directly from a master mold formed with a Tomo process or
with another precision process. FIG. 4B shows the coffin mold
halves assembled together as a flexible wax pattern tool 44 around
a ceramic core, thereby defining an injection cavity 18 conforming
to the desired shape of the subsequently cast metal part 46. Ends
of the ceramic core, known as the core print 48, extend to make
contact with the coffin mold to support the core relative to the
coffin mold and flexible mold insert. The injection cavity is then
filled with wax 50, as illustrated in FIG. 4C, using an injection
process. After the wax hardens, the tool is removed to reveal the
wax pattern 52 shown in FIG. 4D having the desired topography on
its exterior surface. The wax pattern is then coated with ceramic
material (shelled) using techniques known in the art to form the
wax-filled ceramic casting vessel 54 shown in FIG. 4E. The wax is
then removed, such as by heating, to produce the casting vessel 56
shown in FIG. 4F. Molten metal alloy 58 is then cast into the
casting vessel as shown in FIG. 4G, and the ceramic casting vessel
is destructively removed to reveal the component part 46 having an
internal cavity 60 and an integrally cast engineered surface
topography 36 on its surface 38 as illustrated in FIG. 4H.
[0042] The flexible mold inserts of FIG. 4A may be derived directly
from a Tomo process master mold, as described in the cited U.S.
Pat. Nos. 7,141,812 and 7,410,606 and 7,411,204. Alternatively, a
Tomo process mold or other precision master mold may be used to
form one or more intermediate molds, with the intermediate mold(s)
being subjected to a further process step which modifies and
further enhances the surface topography. In one embodiment a metal
foil master Tomo process mold is used to cast a first flexible
mold, and the first flexible mold is used to cast a fibrous
material intermediate mold. The intermediate mold is then grit
blasted to expose some of the fibers at the surface of the mold. A
second flexible mold is then cast into the intermediate mold, and
the second flexible mold will replicate the shape of the exposed
fibers as part of its surface topography. The second flexible mold
is then used in the coffin die of FIG. 4A.
[0043] In its simplest form, the flexible tooling is used to
generate robust features in the surface of the wax pattern that may
generally be recessed into the surface of the wax. Typically, these
would be relatively low angled and of shallow profile with the
objective of creating high angle steps at the edge to create an
interlock geometry and to increase the surface area of the
interface with an overlying coating. A hexagonal type structure or
honeycomb structure may be used. FIG. 5 shows one such surface 62
found to be robust in the surface of a wax pattern using the
above-described steps. Such surfaces in wax patterns produce
translatable honeycomb-like surfaces in investment castings
resulting in a periodically rough surface (in the macro range) that
creates a high degree of interlock and increased surface area for
bond integrity with an overlying coating layer. An additional
benefit may also be gained from increased intermittent coating
thickness across the surface.
[0044] Additional surface engineering can result in even greater
surface area increase and interlock, such as seen in FIG. 6, where
the edges of a hex shape form are rounded out to form gear-cog type
layers 64. Typical surface feature depths have been produced and
shown to be effective at both 0.38 mm and 0.66 mm, but these depths
do not represent optimization and are not meant to be limiting. In
areas of high surface angularity (e.g. leading edge or trailing
edge sections of an airfoil or the airfoil/platform intersection),
pattern protrusions from the surface may be beneficial. Such
protrusions can be produced from second generation flexible molds
(i.e. flexible mold replication from flexible mold masters). FIG. 7
shows an example of a protruded wax surface pattern 66 produced by
such a mold technique. Protruding molds can be engineered to
produce undercuts in the surface, thereby increasing the degree of
mechanical interlock with the coating. This is particularly useful
in highly stressed areas of coatings. It is noted that undercuts
can also be generated in depressed surface features.
[0045] The master tooling can be further modified by non-Tomo
surface modifying techniques such as grit blasting, or sanding, or
producing laser-derived micro pot marks on the surface, or the
addition of a second phase material bonded to the surface of the
master tool, for example with an adhesive such as epoxy. Such
materials may include, without limitation, silicon carbide
particles or chopped fibers which may be applied randomly or with a
predetermined pattern onto the surface. The surface modifying
technique or the second phase material produces a random surface
array on the surface of the tool which can be used to define the
surface of the flexible mold tool and potentially be duplicated
from a second generation flexible mold tool. As an example, FIGS.
8A-8C show wax pattern surfaces 68, 70, 72 produced from a master
tool that was progressively modified with varying degrees of
hybridized surfaces to produce unique micro surface features. In
this case, the master tool was progressively grit blasted, and the
basic Tomo process shape is progressively eroded, resulting in an
ever more rounded structure when progressing from surfaces 68 to
surface 72, but still retaining the basic shape of the Tomo process
feature. This hybridization, combined with the capability of the
Tomo process to produce either recessed or protruding engineered
surfaces, shows the substantial flexibility of the process to
produce a wide variety of engineered surfaces in an as-cast part.
Advantageously, the present process allows for the duplication of a
grit blasted master tool surface through multiple generations of
flexible inserts cast within the grit blasted master tool without
the need for additional actual grit blasting, thus ensuring exact
part-to-part replication. The process effectively becomes
insensitive to surface modification process variation once the
desired master tool surface has been produced because all resulting
surfaces that are derived from the master tool are identical.
[0046] FIGS. 9A-9C illustrate another embodiment for producing an
engineered surface in an investment cast part where the desired
surface finish is too fragile at the wax patterning stage to be
translated effectively into the shell coating 74. Such surfaces
would typically be ones that would result in fragile protrusions in
the wax pattern that would be easily damaged during handling and
shell coating. In this embodiment, a consumable ceramic insert 76
may be formed with a Tomo process or otherwise to have a desired
surface topography 36. The consumable insert forms part of the
flexible wax injection mold 16, as shown in FIG. 9A, but it
detaches from the mold and stays with the wax pattern 52 upon
removal of the mold from the cast wax pattern, as shown in FIG. 9B.
When the wax pattern is shelled and the shell 74 is thermally
treated, the insert remains as part of the shell structure defining
the outer cavity wall 78 for the casting vessel 56, as shown in
FIG. 9C. The inner face of the insert contains the desired
topography of the eventual metal surface of the cast part, and that
detail is retained in a more robust form then with alternative
methods which must translate the topography through the wax. This
process can be used to retain detail in the surface that would
otherwise be compromised in a wax pattern due to fragility. Such a
process lends itself to modularity, such as where additional
anchoring is required for exposed airfoil areas such as leading and
trailing edges of an airfoil. Such ceramic inserts may be partially
thermally processed prior to application to the wax injection
tool.
[0047] FIGS. 10A-10B illustrate another use of a consumable insert
80 for defining a surface opening passageway in the final cast
metal part, such as may be useful for forming trailing edge cooling
passages in a gas turbine blade. The insert may be made of silica,
ceramic or quartz material, and it is designed to fit within a
cooperating recess 82 such as a slot or opening in the flexible wax
pattern mold 16. The insert, the flexible wax pattern mold, and the
core 10 may all be formed with adequate precision, such as with a
Tomo process, so that the projecting legs 84 of the insert abut the
core or mate with a cooperating opening 86 in the core to create a
mechanical interface there between, as shown in FIG. 10A. The
mechanical interface may be a butt joint or a recessed joint or
other cooperating geometry. The insert remains in the wax pattern
52 after the flexible wax pattern mold is removed, as shown in FIG.
10B, and it becomes integral with the shell (not shown) during the
subsequent shell forming process. The projecting legs of the insert
create passageways in the cast metal part between the internal
passageway defined by the core and the exterior surface of the part
defined by the shell inner surface, and they also provide
mechanical support for the core during the wax and metal injection
steps. By forming the flexible wax pattern mold insert with a
precision process such as a Tomo process, it is now possible to
produce blade trailing edge cooling passages with shapes, angles,
aspect ratios, tapers, spirals, etc. that were not previously
possible with prior art techniques. One example is the non-linear
cooling channel to be formed by the insert 80 of FIGS. 10A and 10B.
Advantageously, the insert includes a portion 81 running generally
parallel to a surface of the component, thereby increasing the
effectiveness of the cooling channel. This type of geometry is not
obtainable with standard post-casting machining processes. Each
insert may define a single cooling channel, or alternatively, a
plurality of cooling channels may be defined by an insert 83 formed
with a comb design, as illustrated in FIG. 14.
[0048] FIG. 11 illustrates an embodiment of a coffin mold 40 with a
flexible mold insert 42 wherein the mold and insert are formed with
cooperating alignment features which simplify the placement of the
flexible mold insert into the coffin die and assure proper
alignment there between. FIG. 11 illustrates the use of trapezoidal
shaped protrusions 88 on the surface of the insert and mirror image
shaped grooves 90 on the surface of the coffin die, but one skilled
in the art will appreciate that any variety of cooperating shapes
may be used. One of the advantages of the use of flexible molds is
their low cost and interchangeability, and the use of such
alignment features ensures that each of multiple flexible mold
inserts used with a single coffin die is properly positioned.
Proper positioning of the flexible insert also ensures that the
insert is properly indexed to the core when the core is supported
from the coffin mold.
[0049] A variety of reactive elements may be encased within the
flexible wax injection mold or mold insert. In one example, FIG. 11
illustrates the use of filler particles 92 as a reactive element in
the mold material which is used to form a flexible insert 42. The
filler particles are mixed with the mold material prior to it being
cast into the mold shape while the material is still in a liquid
state. The particles may be any of a variety of materials or
combinations of materials which collectively impart a desired
characteristic to the mold liner. For example, the particles may be
selected to have a desired thermal conductivity characteristic,
such as being highly conductive to heat energy in order to increase
the thermal conductivity of the insert. In other embodiments the
particles may be thermally insulating. At least some of the filler
particles of FIG. 11 may be magnetic and are attracted to magnets
94 mounted in the coffin die, thereby holding the flexible insert
in its proper position within the coffin die. The magnets may be
permanent magnets or electromagnets which further facilitate the
release of the insert from the coffin die when the electromagnets
are de-energized. In another embodiment, magnets are used in the
master mold which is used to cast the flexible mold insert such
that the magnetic particles within the liquid mold material are
attracted toward the magnets while the mold material is curing,
thereby resulting in an preferential distribution of the particles
in regions of the mold proximate the magnets.
[0050] FIG. 12 illustrates the use of a reactive element which is
an active device 96 within the flexible insert 42. The active
device is positioned within the master mold (not shown) during
casting of the mold material such that the device becomes encased
within the mold material. The term "active device" is used herein
to include any object or void other than the mold material which
functions during the use of the flexible mold to enhance the
effectiveness of the mold. Examples of active devices include, but
are not limited to: a sensor such as a temperature or pressure
sensor which may be used to monitor the casting process; an
actuator such as a mechanical vibrator which may be used to
facilitate the flow of casting material throughout the injection
cavity; a temperature regulating device such as a resistance heater
or a fluid channel for the passage of a heating or cooling fluid
which may be used to regulate temperature during a casting process;
etc. The active device may be connected to an associated system 98,
such as an electronic circuit or fluid system located outside of
the mold material, or the device may be isolated within the mold
material and be responsive to a remote communication signal such as
an interrogating RF signal or sound energy.
[0051] As described in the Background of the Invention above, it is
known in the prior art to insert platinum wires (or pins) into the
wax pattern to make contact with the embedded ceramic core after
the wax pattern has been formed. This procedure is precarious,
since insertion of the platinum wires too far can result in damage
to the ceramic core which can remain undetected until after the
metallic part is cast and fails post-casting inspection.
Furthermore, the prior art platinum wires provide no support for
the core during the wax injection step because they are not placed
into position until after the wax is cast. The present invention
contemplates the use of such positioning wires or pins in
conjunction with a flexible wax pattern mold to provide a degree of
flexibility to the support provided by the wires and further to
allow the wires to be positioned against the ceramic core prior to
the wax injection step. FIGS. 13A-13-E illustrates one embodiment
of how this may be accomplished.
[0052] A flexible insert 42 is formed with a surface recess 100 for
receiving a removable supporting element such as a disk 102 as
shown in FIG. 13A. The supporting element may have other shapes in
other embodiments. In one embodiment the flexible insert and disk
may be formed of the same material in order to ensure chemical and
thermal expansion compatibility. The disk is formed with a hole or
opening 104 for receiving a positioning pin 106 such as a known
platinum positioning wire. One skilled in the art will appreciate
that there may be multiple such disks and wires associated with the
insert for support of a particular ceramic core design. A flexible
mold may be formed of a bottom flexible insert (shown) and a top
flexible insert (not shown). The wires, disks and inserts are
preassembled, and then the ceramic core 10 is positioned within the
flexible mold to make light contact with the top surfaces of the
wires. In the horizontal embodiment illustrated, the bottom insert
forms a bed upon which the core is laid, and then the top insert
(not shown) is lowered over the core to form the flexible mold.
Gentle finger pressure may be applied to pre-load the core evenly
onto the wires. The diameter of the hole formed in the disk may be
0.005-0.010 inches undersized compared to the diameter of the wire
in one embodiment to provide a gentle resistance to the movement of
the wire through the disk, thereby allowing the wire to extend into
or through the disk to whatever extent is necessary to support the
core without causing damage to the core material. In an embodiment
where a single flexible insert design is used with multiple core
designs, a blank disk (i.e. no hole 104) may be provided for areas
where there is a recess in the insert but no wire is needed to
support a particular core design.
[0053] It will be appreciated from FIG. 13A that the positioning
wires are in place making proximate contact with the core (i.e.
light contact or close proximity) prior to the wax injection step,
thereby overcoming the prior art problem of proper positioning of
the wires through the cast wax pattern, and also providing a degree
of mechanical support to the core during wax injection, whereas the
prior art use of such pins was for support of the core only during
the subsequent metal casting step. Wax 108 is then injected as
shown in FIG. 13B, and once the wax has solidified, the flexible
insert and positioning disk are removed as shown in FIG. 13C,
revealing the wax pattern 52 and leaving a portion 110 of each wire
extending beyond the wax surface 112. Since each wire may be
positioned to be generally perpendicular to the surface of the core
at that location, there may be multiple pull planes necessary for
removal of the flexible insert from the wires. The tapered shape of
the disk and its cooperating recess in the flexible insert
facilitate the removal of the insert from the plurality of
positioning pins that may be used for a particular core. One may
appreciate that no positioning disk may be necessary for
embodiments where the wires in each mold half are all generally
parallel to each other. In such embodiments, each wire may be
received into a respective hole formed directly into the flexible
insert.
[0054] A ceramic shell coating 74 is then formed onto the wax
pattern by a known dipping process to encompass the protruding
portion of the wires as shown in FIG. 13D, and the wax is then
removed to reveal the completed ceramic casting vessel 56 including
the pre-positioned core support wires, as shown in FIG. 13E.
[0055] FIG. 15 illustrates another embodiment of the use of a core
positioning wire 120 to support a ceramic core 122 within a
flexible wax injection tool 124. In this embodiment, the wire 120
is retained in a hole 126 formed in the surface of the tool 124.
The core 122 is formed to include a pedestal 128 which protrudes
from the median surface 130 of the core to define a valley 132 into
which the wire 120 extends when the core 122 is positioned within
the tool 124. The valley 132 may be formed with sloping walls 134
in order to present a relatively larger opening for receiving the
wire 120 to account for minor positional errors when assembling the
core 122 into the tool 124. While illustrated in cross-section in
FIG. 15, one will appreciate that the walls 134 surround the wire
120 such that mechanical interference between the wire 120 and the
walls 134 of the pedestal 18 will provide support to the core 122
along multiple axes during the wax injection step.
[0056] FIG. 16 illustrates another embodiment of a pedestal 140
formed on a ceramic core 142. This pedestal 140 includes a
protruding undercut 144 which provides mechanical interference with
the head 146 of a pin 148 in two opposed directions along the
longitudinal axis of the pin 148 in order to provide support for
the ceramic core 142 during wax injection when the pin 148 is
engaged with a flexible wax injection tool 150.
[0057] It is noted that any of the wires or pins illustrated in
FIGS. 13A, 15 and 16 may be installed with its longitudinal axis
being perpendicular to the local surface of the tool and
perpendicular to the local surface of the ceramic core, or with its
axis at an angle thereto, since removal of the tool from the pin
after the wax injection step is facilitated by the flexibility of
the tool surface. In this manner, one or a plurality of pins may be
used along a length of the ceramic core to provide mechanical
support in any desired direction, and in particular along a
plurality of non-parallel axes. In some embodiments, it may be
desirable to position the pin at a very high angle relative to the
local surface of the flexible tool, thereby making it difficult to
remove the tool after wax injection in spite of the flexibility of
the tool surface material and in spite of the tapered side surfaces
formed on the disc 102 of FIG. 13A. One such embodiment is
illustrated in FIG. 17 where a pin 160 is positioned within a wax
pattern tool proximate a ceramic core 162 and is supported from a
flexible tool surface 164 at a shallow approach angle A, such as
less than 45 degrees, for example. The pin 160 is held in position
by a supporting element 166 set within a recess 168 having side
surfaces 170 disposed approximately parallel to a pull plane 172
used to remove the tool after the wax injection step. Removal of
the supporting element 166 to expose the end of the pin 160 after
the wax pattern is removed from the tool can be facilitated by
forming the supporting element 166 of a fugitive material, i.e. a
material such as a soluble wax or other material that can be
dissolved from around the pin 160 without destruction of the
non-soluble wax used to cast the wax pattern. Alternatively, the
supporting element 166 may be formed of the same wax as the wax
pattern, and it may be removed by localized melting or mechanical
removal. The supporting element 166 and pin 160 may be pre-formed
as an assembly before being installed into the recess 168, or the
pin 160 may be positioned in the recess 168 in a desired position
and then the fugitive material forming the supporting element 166
may be poured into the recess 168 and then solidified.
[0058] As a result of the increased compliance provided by the
flexible tool described herein, damage to a ceramic core during wax
injection is mitigated when compared to prior art systems which
support the core rigidly from a hard tool. This advantage can be
exploited by injecting the melted wax at higher pressures than are
possible in the prior art. For example, test wax molds have been
injected at 300 psig, which is twice the typical 150 psig of the
prior art, with no damage to the core in 20 of 20 tests, whereas a
300 psig injection pressure using a prior art hard metal tool would
likely destroy the ceramic core in most cases. A higher injection
pressure is economically attractive because it provides better
distribution of the hot wax and results in less preparation being
necessary before gating. It is envisioned within the scope of this
invention that wax injection pressures of greater than 150 psig, or
greater than 175 psig, or greater than 200 psig, or any pressure
than prior art pressures, such as anywhere in the range from 150 to
300 psig or higher may be used.
[0059] The above described investment casting regiment represents a
new business model for the casting industry. The prior art business
model utilizes very expensive, long lead time, rugged tooling to
produce multiple ceramic casting vessels (and subsequently cast
metal parts) from a single master tool with rapid injection and
curing times. In contrast, the new regiment disclosed herein
utilizes a less expensive, more rapidly produced, less rugged
master tool and an intermediate flexible mold derived from the
master tool to produce the ceramic casting vessel with much slower
injection and curing times. Thus, the new casting regiment can be
advantageously applied for rapid prototyping and development
testing applications because it enables the creation of a
first-of-a-kind ceramic casting vessel (and subsequently produced
cast metal part) much faster and cheaper than with the prior art
methods. Multiple different prototype designs may be fabricated
relatively easily from a single master tool by using
interchangeable inserts for design features to be varied.
Furthermore, the new regiment may be applied effectively in high
volume production applications because multiple identical
intermediate flexible molds may be cast from a single master tool,
thereby allowing multiple ceramic casting vessels to be produced in
parallel to match or exceed the production capability of the prior
art methods while still maintaining a significant cost advantage
over the prior art. The time and cost savings of the present
regiment include not only the reduced cost and effort of producing
the master tool, but also the elimination of certain post-metal
casting steps that are necessary in the prior art to produce
certain design features, such as trailing edge cooling holes or
surface roughness, since such features can be cast directly into
the metal part using the new regiment disclosed herein whereas they
require post-casting processing in the prior art. The present
invention provides the potential for an improved yield of
acceptable parts, since it reduces the risk of the placement of
positioning wires against the fragile ceramic core, and it also
provides the potential for higher wax injection pressures without
damage to the ceramic core since the core is supported within the
flexible wax injection mold with more mechanical compliance than is
possible with prior art hard tooling. The present invention not
only produces high precision parts via a flexible mold, but it also
enables part-to-part precision to a degree of that was unattainable
with prior art flex mold processes. Finally, the present regiment
provides these cost and production advantages while at the same
time enabling the casting of design features that heretofore have
not been within the capability of the prior art techniques, thereby
for the first time allowing component designers to produce the
hardware features that are necessary to achieve next generation gas
turbine design goals.
[0060] While various embodiments of the present invention have been
shown and described herein, it will be obvious that such
embodiments are provided by way of example only. Numerous
variations, changes and substitutions may be made without departing
from the invention herein.
* * * * *