U.S. patent application number 10/785669 was filed with the patent office on 2004-08-26 for impregnated ceramic core and method of making.
This patent application is currently assigned to Howmet Research Corporation. Invention is credited to Haaland, Rodney S..
Application Number | 20040166349 10/785669 |
Document ID | / |
Family ID | 25226854 |
Filed Date | 2004-08-26 |
United States Patent
Application |
20040166349 |
Kind Code |
A1 |
Haaland, Rodney S. |
August 26, 2004 |
Impregnated ceramic core and method of making
Abstract
An impregnated fired, porous ceramic core for use in an
investment casting shell mold in the casting of molten metals and
alloys is strengthened by impregnating the core with an aqueous
emulsion of water-insoluble polymer followed by drying to remove
the water.
Inventors: |
Haaland, Rodney S.;
(Morristown, TN) |
Correspondence
Address: |
Edward J. Timmer
P.O. Box 770
Richland
MI
49083-0770
US
|
Assignee: |
Howmet Research Corporation
|
Family ID: |
25226854 |
Appl. No.: |
10/785669 |
Filed: |
February 24, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10785669 |
Feb 24, 2004 |
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09818950 |
Mar 27, 2001 |
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6720028 |
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Current U.S.
Class: |
428/500 ;
427/385.5; 428/688 |
Current CPC
Class: |
C04B 41/009 20130101;
C04B 41/4869 20130101; C04B 2111/00879 20130101; C04B 41/483
20130101; C04B 41/4823 20130101; B22C 9/10 20130101; Y10T 428/31855
20150401; B22C 9/12 20130101; C04B 41/009 20130101; C04B 35/00
20130101; C04B 41/009 20130101; C04B 35/14 20130101 |
Class at
Publication: |
428/500 ;
427/385.5; 428/688 |
International
Class: |
B05D 003/02 |
Claims
I claim:
1. A method of strengthening a fired, porous ceramic core for use
in investment casting, comprising: impregnating the core with an
aqueous emulsion of a water-insoluble polymer, and drying the
impregnated core to remove water.
2. The method of claim 1 wherein the water-insoluble polymer is
selected from the group consisting of acrylic, styrene butadiene,
polyvinyl acetate, styrene acrylic, vinyl acetate acrylic,
vinyl-vinylidene chloride, epoxy, polyvinyl butyrol, and
polyurethane.
3. The method of claim 1 wherein the aqueous emulsion comprises
about 10% to about 60% of said water-insoluble polymer and balance
essentially water.
4. The method of claim 1 wherein the aqueous emulsion comprises
about 10% to 50% by weight of an acrylic polymer and balance
essentially water.
5. The method of claim 4 wherein the acrylic polymer has a glass
transition temperature from 15 to 40 degrees C.
6. The method of claim 4 wherein the aqueous emulsion comprises
about 15% to 30% by weight of an acrylic polymer and balance
essentially water.
7. The method of claim 4 wherein the acrylic polymer is self
cross-linkable.
8. The method of claim 4 wherein the aqueous emulsion includes a
cross linker for the water-insoluble polymer.
9. The method of claim 1 wherein the impregnating of the fired,
porous core is achieved by immersing the core in the aqueous
emulsion.
10. The method of claim 1 wherein the drying of the impregnated
core is achieved by convection at superambient temperature.
11. A method of strengthening a fired, porous ceramic core for use
in investment casting, comprising impregnating the core with an
aqueous emulsion of a water-insoluble acrylic polymer and drying
the impregnated core to remove water.
12. A fired, porous ceramic core for use in investment casting
including water-insoluble polymer in pores of said core.
13. The core of claim 12 wherein the water-insoluble polymer is
selected from the group of consisting of acrylic, styrene
butadiene, polyvinyl acetate, styrene acrylic, vinyl acetate
acrylic, vinyl-vinylidene chloride, epoxy, polyvinyl butyrol, and
polyurethane.
14. The core of claim 13 wherein the core includes cross-linked
acrylic polymer.
15. The core of claim 12 wherein the core includes about 0.2% to
about 5% by weight polymer solids.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to impregnated fired ceramic
cores for use in investment casting of metallic materials and a
method of increasing strength of such cores.
BACKGROUND OF THE INVENTION
[0002] In casting hollow gas turbine engine blades and vanes
(airfoils) using conventional equiaxed and directional
solidification techniques, a fired ceramic core is positioned in an
investment shell mold to form internal cooling passageways in the
airfoil. During service in the gas turbine engine, cooling air is
directed through the passageways to maintain airfoil temperature
within an acceptable range.
[0003] The fired ceramic cores used in investment casting of hollow
turbine engine airfoils typically have an airfoil shape with a
quite thin cross-section trailing edge region. Such ceramic cores
can be prone to distortion and loss of the required dimensional
tolerance during core manufacture and subsequent steps of the
investment casting process such as wax pattern injection about the
fired core and steam autoclaving of the shell mold to selectively
remove the wax pattern.
[0004] Green (unfired) ceramic cores typically are formed to
desired core configuration by injection molding, transfer molding
or pouring of an appropriate ceramic core material that includes
one or more ceramic powders, a fugitive binder such as wax,
polyproplylene, polyolefin, prehydrolized ethyl silicate, and other
additives into a suitably shaped core die. After the green core is
removed from the die, it is subjected to firing at elevated
(superambient) temperature in one or more steps to remove the
fugitive binder and sinter and strengthen the core for use in
casting metallic material, such as a nickel or cobalt base
superalloy. As a result of removal of the binder and fugitive
filler material, if present, the fired ceramic core is porous.
[0005] Attempts have been made to further strengthen the fired,
porous ceramic core. For example, the fired, porous ceramic core
can be impregnated with an aqueous solution of a water-soluble
phenolic formaldehyde resin followed by a 300-400 F. degree oven
bake to set the phenolic resin. Use of the water-soluble phenolic
formaldehyde resin solution as an impregnating medium is
disadvantageous as a result of reduction of the impregnation
strengthening effect imparted by the water-soluble resin in the
presence of atmospheric moisture, such as water and steam, that may
be present in the core manufacturing and foundry environment. That
is, the strengthening effect imparted by the water-soluble resin is
degraded in the presence of atmospheric moisture. The use of the
water-soluble phenolic formaldehyde resin solution as an
impregnating medium is also disadvantageous from the standpoint of
presenting environmental and health concerns with respect to the
formaldehyde resin. Cores impregnated with the water-soluble
phenolic formaldehyde resin solution can exhibit dimensional
distortion during the oven baking operation.
[0006] An object of the present invention is to provide an
impregnated fired ceramic core and method of strengthening the
fired core while overcoming the above-noted disadvantages.
SUMMARY OF THE INVENTION
[0007] An embodiment of the present invention provides an
impregnated fired porous ceramic core for use in an investment
shell mold in the casting of molten metals and alloys wherein the
core is impregnated with an aqueous emulsion of a water-insoluble
polymer followed by drying to remove the water.
[0008] The impregnated fired porous core pursuant to the invention
exhibits a greater strength increase than achieved by the
water-soluble phenolic formaldehyde resin impregnated core after
oven baking, and the strength increase so imparted is more
resistant to degradation in the presence of atmospheric
moisture.
[0009] In a particular embodiment of the present invention, the
water-insoluble polymer is selected from the group consisting of
acrylic, styrene butadiene, polyvinyl acetate, styrene acrylic,
vinyl acetate acrylic, vinyl-vinylidene chloride, epoxy, polyvinyl
butyrol, polyurethane and other water-insoluble polymers.
[0010] A particularly preferred aqueous emulsion comprises about
10% to 50% by weight of an acrylic polymer and balance essentially
water where the acrylic polymer is self cross-linkable. An even
more preferred aqueous emulsion comprises about 15% to 30% by
weight of the acrylic polymer and balance essentially water. The
acrylic polymer preferably has a T.sub.g (glass transition
temperature) from 15 to 40 degrees C. The aqueous emulsion can
include the addition of minor amounts of constituents to reduce
foaming, enhance wetting, and/or improve polymer cross-linking.
[0011] The invention provides a fired, porous ceramic core for use
in investment casting including water-insoluble polymer solids in
pores of the core, the polymer preferably being present in an
amount of about 0.2% to about 5% by weight of the core.
[0012] The above objects and advantages of the present invention
will become more readily apparent from the following detailed
description taken with the following drawings.
DESCRIPTION OF THE DRAWINGS
[0013] The FIGURE is a perspective view of a fired, porous ceramic
core for a gas turbine airfoil that can be made pursuant to the
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0014] The invention provides an impregnated ceramic core
especially useful in casting of hollow gas turbine engine blades
and vanes (airfoils) using conventional equiaxed and directional
solidification techniques. However, the invention is not limited to
ceramic cores for use in investment casting of airfoils as any
other fired, porous ceramic core can be strengthened by practice of
the invention. For purposes of illustration and not limitation, the
FIGURE shows a fired, porous ceramic core 10 for use in investment
casting a hollow gas turbine blade where the core has a
configuration of internal cooling passages to be formed in the
blade casting. The core 10 includes a root region 12 and an airfoil
region 14. The airfoil region 14 includes a leading edge 16 and
trailing edge 18. Openings or slots 20 of various configurations
and dimensions can be provided through the core 10 to form
elongated walls, rounded pedestals and other features in the
interior of the cast blade as well known.
[0015] The ceramic core 10 is formed by preparing a mixture of one
or more suitable ceramic powders (flours), a fugitive binder and
other constituents such as one or more fugitive filler materials,
dispersants, plasticizers, lubricants and other constituents. The
binder can be either an organometallic liquid, such as
prehydrolized ethyl silicate, a thermoplastic wax-based binder, or
a thermosetting resin mixed with the ceramic powder(s) in
appropriate proportions to form a ceramic powder/binder mixture for
molding to shape. The ceramic powders can be blended using a
conventional V-cone blender, pneumatic blender, or other such
blending equipment. The binder can be added using conventional
high-shear mixing equipment at room temperature or elevated
temperature. The ceramic powders may comprise alumina, silica,
zirconia, zircon, yttria, and other powders and mixtures thereof
suitable for casting a particular metal or alloy. U.S. Pat. No.
4,837,187 describes an alumina based ceramic core made from alumina
and yttria flours. The particular ceramic powders, fugitive binder
and other constituents of the ceramic powder/binder mixture form no
part of the invention as conventional ceramic powder and binder
systems can be used to form the ceramic core.
[0016] The ceramic core can be formed by conventional injection
molding, transfer molding, or pouring employed to make green
ceramic cores. For example only, in injection molding a ceramic
core shape, a fluid ceramic powder/binder mixture is injected into
a steel core die having a molding cavity having the core
configuration desired. Injection pressures in the range of 500 psi
to 2000 psi are used to pressurize the fluid ceramic/binder mixture
in the molding cavity defined by the dies. The dies may be cooled,
held at room temperature, or slightly heated depending upon the
complexity of the desired core configuration. After the
ceramic/binder mixture solidifies in the die, the die is opened,
and the green, unfired ceramic core is removed for thermal
processing to remove the fugitive binder and sinter the green
ceramic core to form a fired, porous ceramic core 10 to be used in
the well known lost wax investment casting process. Sintering
achieves consolidation of the ceramic powder particles by heating
to impart strength to the core for use in the investment casting
process. Sintering of the green ceramic core is achieved by means
of heat treatment to an elevated temperature based on the
requirements of the ceramic powders employed. Above U.S. Pat. No.
4,837,187 describes thermal processing of an alumina based ceramic
core. The particular core forming technique, such as injection
molding, transfer molding and pouring, and the particular thermal
processing technique form no part of the invention as conventional
core molding techniques and thermal processing techniques can be
used to make the fired, porous ceramic core which is strengthened
pursuant to the invention.
[0017] The present invention strengthens the fired, porous ceramic
core 10 by impregnating the core with an aqueous emulsion of
water-insoluble polymeric material followed by drying (e.g. in
ambient air at room temperature or using superambient forced air at
for example 200 degrees F.) to remove the water. The impregnated
fired, porous core pursuant to the invention exhibits a greater
strength increase than achieved by the water-soluble phenolic
formaldehyde resin impregnated core after oven baking described
hereabove. Moreover, the strength increase so imparted by practice
of the invention is more resistant to degradation in the presence
of atmospheric moisture as will become apparent.
[0018] The aqueous emulsion of the water-insoluble polymer is
typically produced by commercial manufacturers by a process known
as emulsion polymerization. This process usually involves mixing
water, a liquid monomer, a stablizer, and an initiator under high
shear agitation at a tightly controlled temperature. The result is
a stable suspension of liquid polymeric particles in water. The
aqueous emulsion useful in practice of the invention can comprise a
commercially available aqueous emulsion of the desired polymer
type, solids content and T.sub.g (glass transition temperature). In
an embodiment of the invention, the commercially available aqueous
emulsion can be used as-received or may be diluted with water to
the desired solids content where solids content is a measurement of
polymer content after removing the water from the emulsion.
[0019] The water-insoluble polymer can be selected from the group
consisting of acrylic, styrene butadiene, polyvinyl acetate,
styrene acrylic, vinyl acetate acrylic, vinyl-vinylidene chloride,
epoxy, polyvinyl butyrol, polyurethane and other water-insoluble
polymers.
[0020] When dried to remove all water, the aqueous emulsion useful
in practice of the invention yields a dried polymer having zero or
near zero (e.g. less than 1% by weight) solubility in water at room
temperature (e.g. 20 degrees C.) for 1 hour and less than 5% by
weight in boiling water for 1 hour. For example, when so dried,
Rhoplex HA-16 acrylic latex described below yielded a dried polymer
that exhibited a 0.16% by weight solubility in stirred room
temperature (20 degrees C.) water for 1 hour and a 0.51% by weight
solubility in boiling water for 1 hour where the % by weight is the
percent of the dried polymer that dissolves in water. When so
dried, Rhoplex HA-12 acrylic latex described below yielded a dried
polymer that exhibited a 0.27% by weight solubility in stirred room
temperature (20 degrees C.) water for 1 hour and a 0.77% by weight
solubility in boiling water for 1 hour where the % by weight is the
percent of the dried polymer that dissolves in water. When so
dried, styrene butadiene latex available as Tycac 68010-01 from
Reichold Chemicals Company, Research Triangle Park, N.C. 27709-3582
yielded a dried polymer that exhibited a 0.15% by weight solubility
in stirred room temperature (20 degrees C.) water for 1 hour and a
4.51% by weight solubility in boiling water for 1 hour where the %
by weight is the percent of the dried polymer that dissolves in
water.
[0021] In contrast, when a 10% by weight polyvinyl alcohol (PVA)
solution without cross-linker is dried to remove all water, the
dried PVA not cross-linked exhibited 62.17% by weight solubility in
stirred room temperature (20 degrees C.) water for 1 hour. When a
10% by weight polyvinyl alcohol (PVA) solution with glyoxal
cross-linker is dried to remove all water, the dried PVA
cross-linked using glyoxal exhibited 20.34% by weight solubility in
stirred room temperature water for 1 hour. The dried PVA
cross-linked and not cross-linked with glyoxal both exhibited 100%
solubility in boiling water for 1 hour.
[0022] Advantageously and preferably, the invention can be
practiced using an aqueous emulsion consisting of one or more
water-insoluble polymers and balance essentially water with a minor
amount of a defoaming agent, optional wetting agent (surfactant),
and antimicrobial agent also preferably present. That is, there is
no need for a cross-linker or for a PVA constituent in the aqueous
emulsion in order to achieve the benefits of the invention.
[0023] An aqueous emulsion of the water-insoluble polymer for use
practicing an illustrative embodiment of the invention can comprise
about 10% to about 60% by weight of a water-insoluble polymer and
the balance essentially water.
[0024] An aqueous emulsion pursuant to a preferred embodiment of
the invention comprises about 10% to 50% by weight of one or more
acrylic polymers and balance essentially water with a minor amount
of defoaming agent (e.g. 50 parts per million (ppm) by weight) and
antimicrobial agent (e.g. 50 ppm by weight). An even more preferred
aqueous emulsion comprises about 15% to 30% by weight of one or
more acrylic polymers and balance essentially water with a minor
amount of defoaming agent and antimicrobial agent.
[0025] However, the aqueous emulsion optionally may include up to
10% by weight water-soluble polyvinyl alcohol and a cross linker
for the water-insoluble polymer and water-soluble polymer. A
preferred cross-linker comprises a dialdehyde such as glyoxal
(C.sub.2H.sub.2O.sub.2). Other cross-linkers can be selected from
aldehydes including, but not limited to, dialdehyde,
gluteraldehyde, hydroxyadipaldehyde, thermal setting resins such as
urea-formaldehyde, melamineformaldehyde, polyamide resins, and
salts of multivalent anions. Catalysts optionally can be present in
the aqueous emulsion and can include, but are not limited to,
ammonium nitrate and oxalic acid to accelerate cross-linking
reaction when acrylic latex is used.
[0026] In practicing the invention, a preferred polymer is provided
by an acrylic latex that is self cross-linkable. Such an acrylic
latex is available from Rohm & Haas Company, 100 Independence
Mall West, Philadelphia, Pa., 19106 as Rhoplex HA-16 acrylic latex
having a T.sub.g of 35 degrees C. and % acrylic solids of 45.5% by
weight. This acrylic latex can be purchased in the form of an
aqueous acrylic latex (emulsion). The T.sub.g (glass transition
temperature) of a particular acrylic polymer preferably is the
range of 15 to 40 degrees C. since T.sub.g determines the
temperature at which the dried impregnating material softens.
[0027] Another self-cross-linking acrylic latex that can be used in
the invention is available from Rohm & Haas Corporation as
Rhoplex HA-12 having a T.sub.g of 19 degrees C. and 45.0% by weight
acrylic solids. This acrylic latex can be purchased in the form of
a drum of aqueous acrylic latex (emulsion).
[0028] Table I below describes constituents of three formulations
pursuant to embodiments of the invention offered for purposes of
illustrating but not limiting the invention. In Table I, the
numbers for Examples 1, 2, and 3 represent grams of each
constituent.
1TABLE I Materials Example 1 Example 2 Example 3 Rhoplex HA-16
Acrylic Latex 300 200 100 Water 300 400 369 Airvol 203 PVA 30
Glyoxal crosslinker 1 GEO 8034 Defoamer 0.025 0.025 0.025
[0029] The aqueous emulsion is made by adding the listed acrylic
latex (as-received latex) to distilled water. The optional PVA must
be added to the water as a solid and dissolved prior to adding the
latex. The listed PVA is commercially available as Airvol 203 PVA
from Air Products and Chemicals, Inc., 7201 Hamilton Boulevard,
Allentown, Pa. 18195. The optional cross-linker can be added to the
water in liquid form and is commercially available as Glyoxal 40
cross-linking agent available from Clariet Corporation, 400 Monroe
Road, Charlotte, N.C. 28205. The defoaming agent can be added to
the water in liquid form and is commercially available as GEO 8034
defoamer from GEO Specialty Chemicals Corporation, 701 Wissshickon
Avenue, Cedartown, Ga. 30125. In practice of the invention, an
antimicrobial agent typically also is present and is added to the
water in liquid form and is commercially available as Kathon LX
antimicrobial agent from the Rohm & Haas Company.
[0030] The constituents of the formulation can be mixed with a
conventional propeller mixer for 30 minutes to desired consistency.
The acrylic latex (or other polymer latex) is mixed with an
appropriate amount of water to provide a desired weight percent of
the water-insoluble polymer in the final formulation. The fired,
porous cores can be impregnated by immersion in the aqueous
emulsion of the invention for an appropriate time (e.g. 5
minutes)
[0031] The following Example is offered to further illustrate the
invention.
EXAMPLE 4
[0032] Fired, porous silica-based core testbars having dimensions
of 5 inches length, 1/2 inch width and 1/4 inch thickness were made
by conventional injection molding and transfer molding and then,
after firing, were impregnated with various impregnants pursuant to
the invention and outside the invention as shown in Table II. The
core testbars were impregnated by immersion for 5 minutes in the
impregnant under ambient pressure and air blown with compressed
shop air (e.g. 30 psi air) to remove excess impregnant before
baking or drying. The impregnated core testbars then were tested
for modulus of rupture (MOR) at room temperature using a four point
bending load pursuant to ASTM standard 674-77. An unimpregnated
fired, porous silica-based core testbar was included for
comparison.
[0033] For example, fired, porous silica-based core testbars (12
testbars) were impregnated with an aqueous solution of a
water-soluble phenolic formaldehyde resin (25 weight % resin in
water) outside the invention followed by a 400 F. degree oven bake
for 90 minutes to set the phenolic resin. In Table II, these
testbars are designated "Phenolic Resin".
[0034] Similar fired, porous silica-based core testbars (12) were
impregnated with an aqueous solution of water-soluble Airvol 203
PVA (10 weight % PVA in water) outside the invention and dried by
convection drying at 90 degrees C. for 1 hour. In Table II, these
testbars are designated "10% PVA".
[0035] Similar fired, porous silica-based core testbars (12) were
impregnated pursuant to the invention with an aqueous emulsion
including 22.5% by weight acrylic polymer and balance water (by
diluting Rhoplex HA-16 acrylic latex as in Example 1). The
impregnated testbars were dried by convection drying at 90 degrees
C. for 1 hour. In Table II, these testbars are designated "23%
Acrylic Latex".
[0036] Similar fired, porous silica-based core testbars (12) were
impregnated pursuant to the invention with an aqueous emulsion
including 11.5% by weight acrylic polymer, 5% by weight of water
soluble Airvol 203 PVA and balance water (by diluting Rhoplex HA-16
acrylic latex). The impregnated testbars were dried by convection
drying at 90 degrees C. for 90 minutes. In Table II, these testbars
are designated "11.5% Latex+5% PVA".
[0037] Table II sets forth the MOR results (MOR values set forth in
psi-pounds per square inch).
2TABLE II MOR Values for Core Testbars Impregnated With Various
Impregnants Impregnation Treatment Injection Molded Core Transfer
Molded Core None 1322 2400 Phenolic Resin 1881 2916 11.5% Latex +
5% PVA 3720 3952 23% Acrylic Latex 3668 4339 10% PVA 3743 3904
[0038] It is apparent that impregnation of the fired, porous core
test bars pursuant to the invention (see "23% Acrylic Latex"
testbar and "11.5% Latex+5% PVA" testbar) substantially improved
the MOR as compared to the MOR of the unimpregnated "NONE" core
testbar and the "Phenolic Resin" testbar. The strength increase was
comparable to that achieved by impregnating the testbar with
aqueous solution of water-soluble PVA (see "10% PVA").
[0039] Various impregnated core testbars of the type described
above were subjected to pressurized steam at 250 degrees F. in a
cabinet for 30 minutes to determine resistance to degradation of
impregnated core strength in the presence of steam.
[0040] Table III sets forth the steam exposure results. In Table
III, the "10% PVA" testbar was impregnated with 10% by weight
PVA/water solution as described above. The "10% PVA+Crosslinker"
testbar was impregnated with 10% by weight PVA/0.4% by weight
crosslinker (glyoxal) in aqueous solution in a manner similar to
the "10% PVA" testbar. The "Phenolic Resin" test bar was
impregnated with 25 weight % water-soluble phenolic formaldehyde
resin/water solution as described above. The "23% Acrylic Latex"
testbar was impregnated with an aqueous emulsion including 22.5% by
weight acrylic polymer and balance water as described above
pursuant to the invention.
3TABLE III Percent Reduction in Testbar MOR after Pressurized Steam
Impregnation Treatment Percent Reduction 10% PVA 48.2% 10% PVA +
Crosslinker 20.8% Phenolic Resin 29.5% 23% Acrylic Latex 1.6%
[0041] Table III indicates that the MOR strength of the core
testbar impregnated pursuant to the invention ("23% Acrylic Latex")
exhibited only a 1.6% loss of MOR after steam exposure. In
contrast, the other core testbars impregnated with impregnants
outside the invention exhibited substantial decreases in MOR;
namely, a 20.8% decrease in MOR for the "10% PVA+Crosslinker"
testbar, a 29.5% decrease in MOR for the "Phenolic Resin" testbar,
and a 48.2% decrease in MOR for the "10% PVA" testbar.
[0042] The impregnated fired, porous core testbars pursuant to the
invention exhibited a substantial increase in MOR with the
increased strength imparted to the core being substantially more
resistant to degradation in the presence of atmospheric steam than
the other impregnated testbars outside the invention.
[0043] The invention provides a fired, porous ceramic core for use
in investment casting including water-insoluble polymer solids in
pores of the core. The amount of polymer solids in the core pores
will depend on the density (porosity) of the core. The polymer
solids preferably are present in an amount of about 0.2% to about
5% by weight of the ceramic core. For example, when an acrylic
latex is used as described above, the core will include acrylic
polymer solids in an amount of 0.2 to 5% by weight of the core.
[0044] Although the invention has been described with respect to
certain embodiments thereof, those skilled in the art will
appreciate that the invention is not limited to these embodiments
and changes, modifications, and the like can be made therein within
the scope of the invention as set forth in the appended claims.
* * * * *