U.S. patent application number 10/337799 was filed with the patent office on 2007-07-05 for method of improving the removal of investment casting shells.
Invention is credited to Francois Batllo.
Application Number | 20070151702 10/337799 |
Document ID | / |
Family ID | 32710958 |
Filed Date | 2007-07-05 |
United States Patent
Application |
20070151702 |
Kind Code |
A1 |
Batllo; Francois |
July 5, 2007 |
Method of improving the removal of investment casting shells
Abstract
The removal of an investment casting shell surrounding a
metallic part is improved by adding a salt of alkali or alkaline
earth metal to at least one of the layers of the shell.
Inventors: |
Batllo; Francois; (Burr
Ridge, IL) |
Correspondence
Address: |
NALCO COMPANY
1601 W. DIEHL ROAD
NAPERVILLE
IL
60563-1198
US
|
Family ID: |
32710958 |
Appl. No.: |
10/337799 |
Filed: |
January 7, 2003 |
Current U.S.
Class: |
164/516 ;
164/131; 164/132; 164/519 |
Current CPC
Class: |
B22C 1/02 20130101; B22C
9/04 20130101 |
Class at
Publication: |
164/516 ;
164/519; 164/132; 164/131 |
International
Class: |
B22C 1/02 20060101
B22C001/02; B22D 29/00 20060101 B22D029/00 |
Claims
1-3. (canceled)
4. A method of forming an investment casting shell in a shell
preparation sequence and improving the removal of the investment
casting shell surrounding a metallic part, wherein the shell
preparation sequence comprises deposition of alternating layers of
a refractory slurry and a dry refractory stucco onto a pattern,
wherein an effective amount of the salt of an alkali or alkaline
earth metal is added to at least one layer of the dry refractory
stucco during the shell preparation sequence, and wherein the
refractory slurry layers are free of added alkali or alkaline earth
metal salt during the shell preparation sequence.
5-11. (canceled)
12 The method of claim 4, wherein the salt of the alkali or
alkaline earth metal is selected from the group consisting of:
calcium carbonate, calcium sulfate, calcium magnesium carbonate,
magnesium carbonate, magnesium sulfate, strontium carbonate, and
mixtures thereof.
13. The method of claim 4, including about 1 percent to about 30
percent of the salt of the alkali or alkaline earth metal, based on
weight of the shell.
Description
FIELD OF THE INVENTION
[0001] This invention relates generally to investment casting and,
more particularly, to a method of improving the removal of
investment casting shells.
BACKGROUND OF THE INVENTION
[0002] Investment casting, which has also been called lost wax,
lost pattern and precision casting, is used to produce high quality
metal articles that meet relatively close dimensional tolerances.
Typically, an investment casting is made by first constructing a
thin-walled ceramic mold, known as an investment casting shell,
into which a molten metal can be introduced.
[0003] Shells are usually constructed by first making a facsimile
or pattern from a meltable substrate of the metal object to be made
by investment casting. Suitable meltable substrates may include,
for example, wax, polystyrene or plastic.
[0004] Next, a ceramic shell is formed around the pattern. This may
be accomplished by dipping the pattern into a slurry containing a
mixture of liquid refractory binders such as colloidal silica or
ethyl silicate, plus a refractory powder such as quartz, fused
silica, zircon, alumina or aluminosilicate and then sieving dry
refractory grains onto the freshly dipped pattern. The most
commonly used dry refractory grains include quartz, fused silica,
zircon, alumina and aluminosilicate.
[0005] The steps of dipping the pattern into a refractory slurry
and then sieving onto the freshly dipped pattern dry refractory
grains may be repeated until the desired thickness of the shell is
obtained. However, it is preferable if each coat of slurry and
refractory grains is air-dried before subsequent coats are
applied.
[0006] The shells are built up to a thickness in the range of about
1/8 to about 1/2 of an inch (from about 0.31 to about 1.27 cm).
After the final dipping and sieving, the shell is thoroughly
air-dried. The shells made by this procedure have been called
"stuccoed" shells because of the texture of the shell's
surface.
[0007] The shell is then heated to at least the melting point of
the meltable substrate. In this step, the pattern is melted away
leaving only the shell and any residual meltable substrate. The
shell is then heated to a temperature high enough to vaporize any
residual meltable substrate from the shell. Usually before the
shell has cooled from this high temperature heating, the shell is
filled with molten metal. Various methods have been used to
introduce molten metal into shells including gravity, pressure,
vacuum and centrifugal methods. When the molten metal in the
casting mold has solidified and cooled sufficiently, the casting
may be removed from the shell.
[0008] Investment casting molds must withstand significant
mechanical and drying stresses during their manufacture. Ceramic
shells are designed having high green (air dried) strength to
prevent damage during the shell building process. Once the desired
mold thickness is achieved, it is dewaxed and preheated to
approximately 1800.degree. F. At this point, it is removed from the
high temperature furnace and immediately filled with liquid
(molten) metal. If the mold deforms while the metal is solidifying
(or in a plastic state), the casting dimensions will likely be out
of specification. To prevent high temperature deformation, molds
are designed to have substantial hot strength. Once the casting is
solidified and cooled, low fired strength is desired to facilitate
the knock out or removal of the ceramic mold from the metal
casting.
[0009] Most investment casting molds contain significant quantities
of silica. The silica usually starts as an amorphous (vitreous)
material. Fused silicas and aluminosilicates are the most common
mold materials. When exposed to temperatures above approximately
1800.degree. F., amorphous silica devitrifies (crystallizes)
forming beta cristobalite. Cristobalite has low (alpha) and high
(beta) temperature forms. The beta form has a specific gravity very
close to that of amorphous silica so mold dimensions remain
constant and stresses associated with the phase transformation are
minimal. Upon cooling, beta cristobalite transforms to the alpha
form. This phase transformation is accompanied by an approximate 4%
volume change that creates numerous cracks in the shell, thereby
facilitating mold removal. Cristobalite reduces the fired strength
of silica containing investment casting molds.
[0010] Although investment casting has been known and used for
thousands of years, the investment casting market continues to grow
as the demand for more intricate and complicated parts increase.
Because of the great demand for high-quality, precision castings,
there continuously remains a need to develop new ways to make
investment casting shells more efficiently, cost-effective and
defect-free. For instance, if shell strength was maintained to the
point of metal solidification, followed by a reduction in strength
as the shell cools, improvements in productivity could be realized
through improved knock out (shell removal). This is particularly
desirable for non-ferrous alloys, e.g. alloys of aluminum, copper
and magnesium, because their melting and pouring temperatures are
insufficient to promote cristobalite formation and easy knock
out.
[0011] The knock out is especially difficult when the part presents
a blind hole or a small cavity in which the ceramic is under
compression. The compression occurs during the cooling of the metal
parts, which in general have a higher coefficient of thermal
expansion (CTE) than the ceramic shells. This effect is especially
accentuated in non-ferrous castings because of the high CTE of this
metal (>18 10.sup.-6 m/m).
[0012] Non-ferrous castings produced by investment casters are
rather fragile, so they are cleaned by water or sand blasting,
compared with the aggressive shot blast and vibratory cleaning for
steel and high temperature alloy castings. Residual ceramic on
steel castings is dissolved away using concentrated acids and bases
or molten salt baths. Chemical incompatibility excludes their use
on aluminum and magnesium. If a binder was developed having low
fired strength and associated easy knock out properties upon
exposure to temperatures at or below 1800.degree. F., aluminum
casting cleanup could be greatly improved.
[0013] Accordingly, it would be desirable to provide an improved
method of removing an investment casting shell surrounding a
metallic part.
SUMMARY OF THE INVENTION
[0014] The method of the invention calls for adding a salt of
alkali or alkaline earth metal to at least one of the layers of an
investment casting shell. The addition of a salt of alkali or
alkaline earth metal effectively improves the removal of the
investment casting shell surrounding a metallic part.
DETAILED DESCRIPTION OF THE INVENTION
[0015] The present invention is directed to a method of improving
the removal of an investment casting shell surrounding a metallic
part. In accordance with the invention, a salt of alkali or
alkaline earth metal is added to at least one layer of the
investment casting shell.
[0016] The salt of alkali or alkaline earth metals which may be
used in the practice of the invention include calcium carbonate,
calcium sulfate, calcium magnesium carbonate, magnesium carbonate,
magnesium sulfate, strontium carbonate and mixtures thereof The
preferred salt of alkali or alkaline earth metal for use in
improving the removal of an investment casting shell from a
metallic part is calcium carbonate.
[0017] The salt of alkali or alkaline earth metal can be added to
at least one of the layers of the investment casting shell by any
conventional method generally known to those skilled in the art. In
a preferred embodiment, the salt of alkali or alkaline earth metal
is added to at least one layer of the refractory stucco. However,
in the practice of the present invention, the salt of alkali or
alkaline earth metal may alternatively be added to at least one
layer of the refractory slurry or to at least one layer of both the
refractory slurry and the refractory stucco.
[0018] The salt of alkali or alkaline earth metal is used at a
concentration that will effectively improve the removal of an
investment casting shell surrounding a metallic part. It is
preferred that the amount of salt of alkali or alkaline earth metal
which is added to at least one layer of the shell be in the range
of about 1 to about 30% by weight of the shell. More preferably,
the amount of salt of alkali or alkaline earth metal is from about
5 to about 25%, with about 8 to about 20% being most preferred.
[0019] The present inventor has discovered that adding a salt of
alkali or alkaline earth metal to at least one layer of an
investment casting shell effectively improves the removal of the
shell surrounding the metallic part.
EXAMPLES
[0020] The following examples are intended to be illustrative of
the present invention and to teach one of ordinary skill how to
make and use the invention. These examples are not intended to
limit the invention or its protection in any way.
Example 1
CaCO.sub.3 Mixed in Refractory Slurry
[0021] Slurries were prepared using the following formulas:
TABLE-US-00001 TABLE 1 Slurry Ingredients Concentrations (ratios)
Colloidal silica.sup.1 1920 g Deionized water 386 g Latrix .RTM.
6305 polymer.sup.2 138 g Nalcast .RTM. P1 (-200 mesh) fused
silica.sup.3 1200 g Nalcast .RTM. P2 (-120 mesh) fused silica.sup.4
3600 g Nalco .RTM. 8815 anionic wetting agent.sup.5 1.0 g Dow
Corning .RTM. Y-30 antifoam.sup.6 4.0 g CaCO.sub.3 fine Marblemite
.RTM..sup.7 0.0 g (0%), 288 g (6%), 576 (12%), 864 g (18%), 1200 g
(25%) CaCO.sub.3 coarse 40-200.sup.8 480 g (10%), 960 (20%), 1680 g
(30%) .sup.1Nalcoag .RTM. 1130 (8 nanometer, sodium stabilized)
(available from Ondeo Nalco Company) .sup.2Styrene butadiene latex
at 10% based on diluted colloidal silica (available from Ondeo
Nalco Company) .sup.3Available from Ondeo Nalco Company
.sup.4Available from Ondeo Nalco Company .sup.570% sodium dioctyl
sulfosuccinate (available from Ondeo Nalco Company) .sup.630%
silicone emulsion (available from Dow Corning Corporation of
Midland, Michigan) .sup.7Cultured marble calcium carbonate
(available from Imerys Corporation of Roswell, Georgia)
.sup.8Screened grade calcium carbonate (available from Imerys
Corporation of Roswell, Georgia)
[0022] After seventy-two hours of mixing, the viscosities of the
slurries were measured and adjusted using a number four Zahn cup.
The viscosities ranged from 14-18 seconds. Minor binder additions
(colloidal silica+water+polymer) were made to obtain the desired
rheology. Once adjusted, the slurries were ready for dipping.
[0023] Wax patterns were cleaned and etched using Nalco.RTM. 6270
pattern cleaner (available from Ondeo Nalco Company) followed by a
water rinse. Wax bars were dipped into each slurry followed by
Nalcast.RTM. S1 and S2 fused silica stucco (available from Ondeo
Nalco Company) (applied by the rainfall method). Dry times started
at 1.5 hours and progressed up to 3.5 hours as coats were added.
The final shells had two coats with Nalcast.RTM. S1 stucco, three
coats with Nalcast.RTM. S2 stucco plus one seal coat (no stucco).
All coats were dried at 73-75.degree. F., 35-45% relative humidity
and air flows of 200-300 feet per minute. After a twenty-four hour
final dry, the shells were placed into a desiccator for an
additional twenty-four hours prior to testing.
[0024] Several shell properties were evaluated using modulus of
rupture (MoR) bars prepared from the experimental slurries. The
bars were broken with a three point bending fixture on an ATS
universal test machine (available from Applied Test Systems, Inc.
of Butler, Pa.). The analog output (voltage) was fed into a
personal computer containing an analog-to-digital conversion board
and data acquisition software. The data was stored as a load versus
time, or load versus displacement plot. Calculations and analyses
were performed using data acquisition software or spreadsheet
programs. The following physical properties were determined for the
MoR specimens:
Fracture Load
[0025] The fracture load is the maximum load that the test specimen
is capable of supporting. The higher the load, the stronger the
test specimen. It is affected by the shell thickness, slurry and
shell composition. This property is important for predicting shell
cracking and related casting defects. The fracture load is measured
and recorded for test specimens in the green (air dried), fired
(held at 1800.degree. F. for one hour and cooled to room
temperature) and hot (held at 1800.degree. F. for one hour and
broken at temperature) condition. Results are normalized and
expressed as an Adjusted Fracture Load (AFL). The AFL is simply the
fracture load divided by the specimen width for a two inch test
span.
Shell Thickness
[0026] Shell thickness is influenced by slurry and shell
composition, combined with the shell building process. Thickness
fluctuations are indicative of process instability. Non-uniform
shell thickness creates stresses within the shell during drying,
dewaxing, preheating and pouring. Severe cases lead to mold
failure. The mold surrounds and insulates the cooling metal.
Changes in thickness can affect casting microstructure, shrinkage,
fill and solidification rates.
Modulus of Rupture
[0027] A flat ceramic plate is prepared using a rectangular wax bar
as the pattern. Typical dimensions are 1.times.8.times.1/4 inches.
The bar is invested using the desired shell system. After drying,
the edges are removed with a belt sander. The two remaining plates
are separated from the wax, yielding two test specimens. The
specimens are broken using a three point loading apparatus on an
ATS universal test machine. MoRs are calculated for bars in the
green, fired and hot conditions. MoR = 3 .times. PL 2 .times. bh 2
##EQU1## where P=Fracture load in pounds [0028] L=Specimen length
in inches (distance between supports) [0029] b=Specimen width at
point of failure in inches [0030] h=Specimen thickness at point of
failure in inches
[0031] The MoR is a fracture stress. It is influenced by fracture
load and specimen dimensions. Shell thickness is of particular
importance since the stress is inversely proportional to this value
squared. The uneven nature of the shell surface makes this
dimension difficult to accurately measure, resulting in large
standard deviations. This deficiency is overcome by breaking and
measuring a sufficient number of test specimens.
Bending or Deflection
[0032] The test specimen bends as the load is applied. The maximum
deflection is recorded as the specimen breaks. Bending increases
with flexibility and polymer concentration. A flexible shell is
capable of withstanding the expansion and contraction of a wax
pattern during the shell building process. Bending is measured for
bars in the green condition.
Fracture Index
[0033] The fracture index is a measure of the work or energy
required to break a shell in the green condition. It is indicative
of shell "toughness", i.e., the higher the index, the tougher the
material. For example, a polypropylene bottle is "tougher" than a
glass bottle and therefore has a higher fracture index. The index
is an indicator of crack resistance. High index shells require more
energy to break them than low index systems.
[0034] The fracture index is influenced by slurry and shell
composition. Polymer additives increase the index. Soft polymers
produce higher index shells than stiff ones. The index is
proportional to shell flexibility. A shell that is capable of
yielding absorbs more energy than a rigid, brittle one.
[0035] The fracture index is determined by integrating the area
beneath the load/displacement curve for a MoR test specimen. The
index measures (force).times.(distance) when monitoring
displacement or (force).times.(time) when monitoring load time. To
convert from (force).times.(time) to (force).times.(distance), the
loading rate is used. Test results are normalized by simply
dividing the index value by the specimen width for a two inch test
span.
Modulus of Elasticity
[0036] The modulus of elasticity, also known as Young modulus, MoE
or elastic modulus is a proportionality constant obtained from the
stress-strain curve. It can be calculated from MoR (bending)
analysis. The modulus of elasticity is a measure of stiffness of
the material. A stiff, rigid material will display a strain/stress
curve with a steep slope and high MoE. A soft, flexible material
will exhibit a strain/stress curve with a flatter slope and low
MoE. The modulus of elasticity is independent of sample dimension.
It provides an accurate mean for comparing different shell
systems.
[0037] As shown below in Table 2, the fired strength decreased with
the amount of fine or coarse CaCO.sub.3 in the shell. The best
system was achieved with the addition of 10-20% coarse CaCO.sub.3,
which showed approximately a 30% decrease in fired MoR with a
minimal effect on the green properties of the shell. TABLE-US-00002
TABLE 2 Summary green data MoR % CaCO.sub.3 (psi) AFL (lbs) MoE
(kpsi) F Index Bending (mils) Fine 0% 556.73 9.02 381 26.43 4.77 6%
448.06 8.20 231 28.72 5.46 12% 406.47 7.96 228 28.11 5.03 18%
353.20 7.51 185 26.25 5.29 30% 261.82 6.26 115 23.59 5.75 Coarse
10% 588.83 8.29 346 31.26 5.44 20% 482.67 9.00 253 31.57 5.37 30%
385.94 9.46 186 34.70 5.33 Summary fired data MoR (psi) AFL (lbs) %
CaCO.sub.3 Fine 0% 1016.82 15.33 6% 804.23 14.82 12% 651.27 12.59
18% 504.93 11.54 25% 397.56 9.05 % CaCO.sub.3 Coarse 10% 752.87
13.91 20% 738.12 15.11 30% 530.53 11.87
Example 2
CaCO.sub.3 Added as Discrete Dry Refractory (Stucco) Coats
[0038] Slurries were prepared using the following formulas:
TABLE-US-00003 TABLE 3 Slurry Ingredients Concentrations (ratios)
Colloidal silica 1920 g Deionized water 401 g TX-11280 polymer 138
g Nalcast .RTM. P1 (-200 mesh) fused silica 1200 g Nalcast .RTM. P2
(-120 mesh) fused silica 3600 g Nalco .RTM. 8815 anionic wetting
agent 2.2 g Dow Corning .RTM. Y-30 antifoam 4.0 g Alternative
layers of CaCO.sub.3 and SiO.sub.2 stucco were used during the
shell preparation sequence as follows: Stucco Stucco Stucco Coat
Coat Coat Stucco Coat Stucco Coat Formulation # #1 #2 #3 #4 #5 1 S1
S1 S2 S2 S2 2 S1 S1 CaCO.sub.3.sup.9 S2 S2 3 S1 S1 CaCO.sub.3 50/50
S2 + S2 CaCO.sub.3 4 S1 S1 CaCO.sub.3 CaCO.sub.3 S2 5 S1 S1
CaCO.sub.3 CaCO.sub.3 CaCO.sub.3 The shell test methods were also
the same. .sup.9Screened grade calcium carbonate (available from
Imerys Corporation of Roswell, Georgia)
[0039] As shown below in Table 4, the addition of the CaCO.sub.3 in
the stucco layer decreased fired shell strength without
compromising green strength. The best results were obtained with 1
CaCO.sub.3 stucco layer showing a 35% fired strength decrease with
a minimal effect on hot strength. The green, hot and fired MoR
results for shells with and without CaCO.sub.3 stucco coat were as
follows: TABLE-US-00004 TABLE 4 Summary green data MoR Bending
Formulation # (psi) AFL (lbs) MoE (kpsi) F Index (mils) 1 434.51
9.38 175 39.17 6.42 2 470.52 7.11 238 28.64 6.30 3 504.09 5.95 294
23.10 6.53 4 473.57 5.30 284 22.07 6.34 5 455.91 4.40 292 18.92
6.18 Summary hot and fired data Hot Fired Formulation # MoR (psi)
MoR (psi) Hot AFL (lbs) Fired AFL (lbs) 1 1204.13 707.82 19.80
14.36 2 1023.13 465.12 13.14 7.40 3 619.81 377.00 9.05 4.96 4
700.56 254.34 7.57 3.09 5 518.13 131.40 4.65 1.38
Example 3
CaCO.sub.3 Mixed with Dry Refractory Stucco
[0040] Slurries were prepared using the following formulas:
TABLE-US-00005 TABLE 5 Slurry Ingredients Concentrations (ratios)
Colloidal silica 1920 g Deionized water 401 g TX-11280 polymer 138
g Nalcast .RTM. P1 (-200 mesh) fused silica 1200 g Nalcast .RTM. P2
(-120 mesh) fused silica 3600 g Nalco .RTM. 8815 anionic wetting
agent 2.2 g Dow Corning .RTM. Y-30 antifoam 4.0 g Blends of
SiO.sub.2 and CaCO.sub.3 were used as stucco coats during the shell
preparation sequence as follows: Stucco Stucco Stucco Formu- Coat
Coat Coat Stucco Coat Stucco Coat lation # #1 #2 #3 #4 #5 1 S1 S1
S2 S2 S2 2 S1 S1 S2 + 5% S2 + 5% CaCO.sub.3 S2 + 5%
CaCO.sub.3.sup.10 CaCO.sub.3 3 S1 S1 S2 + 10% S2 + 10% CaCO.sub.3
S2 + 10% CaCO.sub.3 CaCO.sub.3 4 S1 S1 S2 + 20% S2 + 20% CaCO.sub.3
S2 + 20% CaCO.sub.3 CaCO.sub.3 5 S1 S1 S2 + 30% S2 + 30% CaCO.sub.3
S2 + 30% CaCO.sub.3 CaCO.sub.3 .sup.10Screened grade calcium
carbonate (available from Imerys Corporation of Roswell,
Georgia)
[0041] An erosion test was added to fully characterize the benefit
of the CaCO.sub.3 addition on the knock out properties of the
shells. The new technique simply consisted of a simulation of the
sand blasting process used in the industry. Shell coupons of
dimensions: L, W=0.75'' to 1.5'' and 0.4'' thick max were placed in
a fixture located on one of the walls of the blasting cabinet. A
spring blade held the coupon against a stainless steel mask with a
0.5 x 0.5'' window. This mask ensured that identical surface area
was exposed to the blast from one sample to the next and a
homogenous blasting media flowed across the surface. The thickness
of the coupon was measured prior to the test. The coupon was then
exposed to the blasting media until it was perforated. The time to
perforation was measured and an erosion speed was calculated for
each sample. The test was repeated with a representative number of
coupons to allow an accurate determination of the erosion
speed.
[0042] As shown below in Table 6,,the addition of the CaCO.sub.3
blended with SiO.sub.2 in the stucco layer decreased fired shell
strength without compromising green strength. The best results were
obtained with 10-20% CaCO.sub.3 addition to the stucco layer
showing a 20 to 40% fired strength decrease with a minimal effect
on hot strength. The erosion speed data show that a five fold
increase in erosion speed was achieved after 24 and 48 hours
cooling shells containing 8 and 10% CaCO.sub.3. The green, hot,
fired MOR and erosion test results for shells with and without
CaCO.sub.3 stucco coat were as follows: TABLE-US-00006 TABLE 6
Summary Green Data F % CaCO.sub.3 MoR (psi) AFL (lbs) MoE (kpsi)
Index Bending (mils) 0 564.40 9.44 268 38.38 6.32 5 566.48 9.11 306
33.16 5.78 10 580.84 8.64 305 32.67 6.02 20 577.19 8.64 305 32.67
6.02 30 559.01 6.71 355 24.90 5.62 Summary hot and fired data Fired
% CaCO.sub.3 Hot MoR (psi) MoR (psi) Hot AFL (lbs) Fired AFL (lbs)
0 1326.26 932.15 19.28 16.22 5 1127.35 809.54 15.44 11.94 10 923.31
729.07 15.44 11.14 20 723.13 558.85 9.75 6.54 30 793.39 496.62 7.81
5.59 Summary erosion data 2 Hr cooling 24 Hr cooling 48 Hr cooling
% CaCO.sub.3 (mils/sec) (mils/sec) (mils/sec) 0 4.89 4.40 4.79 2
4.51 5.13 6.38 4 4.37 10.74 9.64 6 4.46 10.71 14.71 8 4.34 14.70
25.10 10 4.23 10.58 24.28
[0043] While the present invention is described above in connection
with preferred or illustrative embodiments, these embodiments are
not intended to be exhaustive or limiting of the invention. Rather,
the invention is intended to cover all alternatives, modifications
and equivalents included within its spirit and scope, as defined by
the appended claims.
* * * * *