U.S. patent application number 10/964903 was filed with the patent office on 2006-04-20 for method of improving the removal of investment casting shells.
Invention is credited to Francois Batllo, Jeffrey S. Mosher, David S. Viers.
Application Number | 20060081350 10/964903 |
Document ID | / |
Family ID | 36179513 |
Filed Date | 2006-04-20 |
United States Patent
Application |
20060081350 |
Kind Code |
A1 |
Batllo; Francois ; et
al. |
April 20, 2006 |
Method of improving the removal of investment casting shells
Abstract
A method of improving the removal of an investment casting shell
surrounding a metallic article. The shell is made from the deposit
of layers refractory slurry and stucco onto a pattern. The shell
contains, after firing, an amount of an alkali metal oxide or an
alkaline earth metal oxide sufficient to reduce the strength of the
shell. By this invention, after firing of the shell and pouring of
the molten metallic part into the shell, the shell is hydrated to
further reduce the strength of the shell, to facilitate its removal
from the metallic part.
Inventors: |
Batllo; Francois; (Burr
Ridge, IL) ; Viers; David S.; (Aurora, IL) ;
Mosher; Jeffrey S.; (Lakewood, OH) |
Correspondence
Address: |
NALCO COMPANY
1601 W. DIEHL ROAD
NAPERVILLE
IL
60563-1198
US
|
Family ID: |
36179513 |
Appl. No.: |
10/964903 |
Filed: |
October 14, 2004 |
Current U.S.
Class: |
164/131 ;
164/132; 164/516 |
Current CPC
Class: |
B22D 29/00 20130101;
B22C 1/02 20130101 |
Class at
Publication: |
164/131 ;
164/516; 164/132 |
International
Class: |
B22D 29/00 20060101
B22D029/00; B22C 9/04 20060101 B22C009/04 |
Claims
1. A method of improving the removal of an investment casting shell
surrounding a metallic article, said shell being made from the
deposit of layers of refractory slurry and stucco onto a pattern,
and containing, after firing, an amount of an alkali metal oxide or
an alkaline earth metal oxide sufficient to reduce the strength of
the shell, which comprises the steps of: firing the shell, pouring
the molten, metallic part into the shell, and then hydrating the
shell, whereby the strength of the shell is further reduced to
facilitate its removal from the metallic part.
2. The method of claim 1 in which said alkaline earth metal oxide
is present.
3. The method of claim 2 in which the alkaline earth metal oxide
comprises calcium oxide.
4. The method of claim 1 in which said hydrating of the shell is
performed by a water spray or dip.
5. The method of claim 1 in which said hydrating of the shell is
performed by exposure to air of at least about fifty percent
relative humidity.
6. The method of claim 1 in which said alkali metal oxide or
alkaline earth metal oxide is formed in situ in the shell by heat
decomposition of an alkali metal carbonate or alkaline earth metal
carbonate.
7. The method of claim 6 in which said alkaline earth metal
carbonate is present in at least one layer used to form said
shell.
8. The method of claim 6 in which said alkaline earth metal
carbonate is present in at least one layer of stucco used to form
said shell.
9. The method of claim 1 in which an amount of 1 to 40 weight
percent of said alkali metal oxide or said alkaline earth metal
oxide is present, based on the weight of the total shell.
10. The method of claim 31 in which an amount of about 8 to 25
weight percent of said alkali earth metal or alkaline earth metal
is present, based on the weight of the total shell.
11. The method of claim 1 in which at least one layer of said shell
contains from about 25 to 75 weight percent of said alkali metal
oxide or alkaline earth metal oxide.
12. The method of claim 1 in which said hydrating of the shell
takes place with humid air at an air temperature of at least about
50.degree. C.
13. The method of claim 1 in which said alkaline earth metal oxide
is formed in situ in the shell by heat decomposition of calcium
carbonate.
14. The method of claim 1 in which said alkali metal oxide or
alkaline earth metal oxide has a first average grain size, and is
present in at least one layer of said stucco, said stucco also
containing a finely divided refractory material having a second
average grain size that is no more than 50% greater or less than
the first average grain size.
15. The method of claim 14 in which said refractory material
comprises fused silica.
16. The method of claim 14 in which said refractory material
comprises aluminosilicate.
17. The method of claim 14 in which said refractory material
comprises zircon.
18. The method of claim 14 in which said first average grain size
is 30-50 mesh.
19. The method of claim 1 in which the shell is fired at a
temperature above about 1100.degree. F. and less than about
1800.degree. F.
20. A method of improving the removal of an investment casting
shell surrounding a metallic article, said shell being made from
the deposit of layers of refractory slurry and stucco onto a
pattern, and containing, after firing, an amount of calcium oxide
or magnesium oxide sufficient to reduce the strength of the shell,
which comprises the steps of: firing the shell at a temperature
above about 1100.degree. F. and less than about 1800.degree. F.,
pouring the molten, metallic article into the shell, and then
hydrating the shell, whereby the strength of the shell is further
reduced to facilitate its removal from the metallic part.
21. The method of claim 20 in which said hydrating of the shell is
performed by exposure to air of at least about 50% relative
humidity.
22. The method of claim 20 in which calcium oxide is present in
said shell.
23. The method of claim 20 in which said calcium oxide or magnesium
oxide is formed in situ in the shell by heat decomposition of a
calcium or magnesium salt.
24. The method of claim 1 in which a calcium or magnesium salt is
present in at least one layer of a refractory stucco used to form
said shell.
25. The method of claim 20 in which said magnesium or calcium oxide
has a first average grain size, and is present in at least one
layer of said stucco, said stucco also containing a finely divided
refractory material having a second average grain size that is no
more than about 50% greater or less than the first average grain
size.
26. The method of claim 25 in which said refractory material is
fused silica and the first average grain size is 30-50 mesh.
27. A method of improving the removal of an investment casting
shell surrounding a metallic part, said shell being made from the
deposit of layers of refractory slurry and stucco onto a pattern,
and containing, after firing, an amount of calcium oxide sufficient
to reduce the strength of the shell, which comprises the steps of:
firing the shell, pouring the molten metallic part into the shell,
and then hydrating the shell, said calcium oxide having a first
average grain size and being present in at least one layer of said
stucco, said stucco also containing a finely divided refractory
material having a second average grain size that is no more than
about 50% greater or less than the first average grain size,
whereby the strength of the shell is further reduced to facilitate
its removal from the metallic part, and then hydrating the shell,
whereby the strength of the shell is further reduced to facilitate
its removal from the metallic part.
28. The method of claim 27 in which said refractory material
comprises fused silica and the first average grain size is 30-50
mesh.
29. The method of claim 27 in which the shell is fired at a
temperature above about 1100.degree. F.
30. The method of claim 1 in which said alkali metal oxide or
alkaline earth metal oxide is formed in situ in the shell by heat
decomposition of alkali earth metal or alkaline earth metal is
selected from the group consisting of: calcium carbonate, calcium
sulfate, calcium magnesium carbonate, magnesium carbonate,
magnesium sulfate, strontium carbonate, sodium carbonate, potassium
acetate, and mixtures thereof.
31. The method of claim 1 in which said alkali metal oxide or
alkaline earth metal oxide is formed in situ in the shell by heat
decomposition of an alkali earth metal or alkaline earth metal,
respectively.
32. The method of claim 31 in which an amount of about 5 to 40
weight percent of said alkali earth metal or alkaline earth metal
is present, based on the weight of the total shell.
33. The method of claim 14 in which said refractory material is
selected from the group consisting of: fused silica; quartz;
alumina; aluminosilicate; zircon; and zirconia.
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. 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.
[0003] 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.
[0004] The steps of dipping the pattern into a refractory slurry to
form a layer, and then sieving onto the freshly dipped pattern dry,
refractory grains as an added "stucco" layer, 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.
[0005] 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.
[0006] 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.
[0007] The complete removal of the cast or shell surrounding the
part, which is sometimes called the "knock-out", often requires
long, dirty, dangerous, and costly processes, using sandblasting,
mechanical vibration, or caustic baths.
[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 phase transition 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, more 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 coefficient
of thermal expansion of this metal (>18.times.10.sup.-6m/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 castings. 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.
[0014] Batllo U.S. patent application Ser. No. 10-337,799 addresses
this issue by 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 by reducing the shell strength, particularly after
firing.
[0015] Such addition of alkaline earth metal salt, such as calcium
carbonate added to the "green" investment casting shell, can
provide a weakening effect that results in a two to threefold
increase in erosion speed of the fired shell that surrounds the
solidified metal casting, as measured by blasting tests such as
sandblasting.
SUMMARY OF THE INVENTION
[0016] By this invention, a further improvement of the method of
removal of an investment casting shell surrounding a metallic
article is provided. The improvement is effective for investment
casting shells of the type disclosed in the Batllo U.S. patent
application cited above, the shells being made by depositing
respective layers of refractory slurry and stucco onto a pattern,
and containing, after firing, an amount of an alkali metal oxide or
alkaline earth metal oxide sufficient to reduce the strength of the
shell. By this invention, the shell is fired in generally
conventional manner, typically above 1100.degree. F., and ferrous
or nonferrous, molten metal is poured into the shell to form the
metallic article therein. Following this, the shell is hydrated,
whereby the strength of the shell is further reduced, to facilitate
its removal from the metallic part by breakaway. The hydration step
takes place generally after the molten metal has solidified in the
shell, so that the shell is ready to be removed.
[0017] Typically, an alkaline earth metal oxide may be used as the
shell-weakening ingredient, particularly calcium oxide, which may
be initially added to the shell before firing in the form of a salt
such as a calcium salt, calcium carbonate, as an ingredient in the
stucco, for example, forming calcium oxide during firing. In
another embodiment, a magnesium salt may be utilized. The salt of
alkaline metal or alkaline earth metal is added to at least one
layer of the investment casting shell to provide the desired
weakening effect, which weakening effect is typically magnified
upon firing of the shell, and then further magnified by the
hydration step, so that the shell is more easily removed from the
casting.
[0018] The salts of alkali or alkaline earth metals which may be
used in the practice of the invention to form the investment
casting shell include, among others, calcium carbonate, calcium
sulfate, calcium magnesium carbonate, magnesium carbonate,
magnesium sulfate, strontium carbonate, sodium carbonate, potassium
acetate, and mixtures thereof. A 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.
Oxides are then generally formed on firing.
[0019] The salt of alkali or alkaline earth metal can be added to
at least one of the layers of the investment casting shell. In a
preferred embodiment, the salt of alkali metal or alkaline earth
metal is added to at least one layer of the refractory stucco,
which is typically added as a dry powder sticking to a slurry layer
previously placed on the shell. 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.
[0020] The hydration step may be performed after the metal casting,
contained in the shell after pouring, has solidified. The hydration
may be performed by a water spray, dipping of the shell in water,
or a similar process. Effective hydration can also take place by
exposure of the metal-filled shell to air, typically of at least
about 50% relative humidity at elevated temperatures, when
acceleration of the process is desired. Such humid air can greatly
accelerate hydration, so it is generally preferred for the
hydration step using humid air to take place at an elevated shell
temperature of at least about 100.degree. C. and at least 50%
relative humidity.
[0021] As stated above, alkali metal salts and alkaline earth metal
salts may be added as ingredients to the slurry and stucco
formulations used to make the various layers of the shell, with the
metal salts being included in at least one such layer and
optionally any number or all of the layers. One suitable alkaline
earth metal salt is the carbonate salt, particularly calcium
carbonate. Then, upon firing, the alkali metal salt or alkaline
earth metal salt which is used is converted, typically for the most
part, to alkali metal oxide or alkaline earth metal oxide, which is
formed in situ in the shell by heat decomposition. Alternatively,
the alkali metal oxide or alkaline earth metal oxide itself may be
directly added during the shell making process.
[0022] Typically, an amount of about 1-40 weight percent of the
alkali metal oxide or alkaline earth metal oxide may be present,
based on the weight of the shell.
[0023] The salt of alkali metal or alkaline earth metal is used at
a concentration that will effectively improve the removal of an
investment casting shell surrounding a metallic part by weakening
it. In some embodiments, the amount of salt of alkali or alkaline
earth metal is from about 5 to about 40%, with about 8 to about 25%
being typically in some embodiments, based on the weight of the
shell.
[0024] Typically, at least one layer of the shell contains about 25
to 70 or 75 weight percent of the alkali metal oxide or alkaline
earth metal oxide, based on the individual layer material.
[0025] Furthermore, the alkali metal oxide or alkaline earth metal
oxide may, in some preferred embodiments, be present in at least
one layer of the stucco material, and may have a first average
grain size. The stucco also contains a finely divided refractory
material such as fused silica, quartz, alumina, aluminosilicate,
zircon, zirconia or another known material used in this manner. The
finely divided refractory material preferably has a second grain
size that is typically no more than 50% greater or less, than the
first average grain size.
[0026] The effect of weakening on hydration can be magnified by
this manner. The particles of alkali metal oxide or alkaline earth
metal oxide will tend to swell upon hydration, and increased
mechanical deformation can take place in the fired, hydrated
investment casting shell which can cause the generation of cracks
and the further reduction of strength in the shell.
[0027] For example, the first average grain size may be 30-50
mesh.
[0028] Also, as previously discussed, the shells that are used to
form aluminum, copper, and magnesium castings, among others,
typically have melting and pouring temperatures that are
insufficient to promote cristobalite formation. Thus, the shells
remain unduly strong after firing, and are difficult to remove from
the castings.
[0029] Further advantages achieved in accordance with this
invention, when the shell is fired at a temperature of less than
about 1800.degree. F., as is typically the case relative to the
nonferrous metals mentioned above. The shell can be weakened
through hydration, even though Cristobalite phase transition does
not take place.
[0030] The adding of 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 by weakening
the shell, after it is conventionally fired. By this invention,
further improvement is provided by hydration of the shell after the
molten metal casting has hardened.
DESCRIPTION OF DRAWING
[0031] FIG. 1 is a graph showing the effect of hydration by humid
air on the strength of shell wall material.
DESCRIPTION OF THE SPECIFIC EMBODIMENTS
[0032] 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
[0033] To determine the effect of hydration on a fired wall of
material used to make investment casting shells, rectangular wax
bars were dipped into a priming slurry of commercial type
comprising: 37 gallons of a pre-blend binder (95% Nalco 6010 finely
divided silica with 5% of Nalco 6300 organic polymer); 300 ml of
Nalco 8815 wetting agent; 200 ml of an anti-foam agent; 500 lbs. of
200 mesh fused silica flour; and 500 lbs. of 200 mesh zircon
flour). The wax bar made of this material was dipped into the
slurry along most of its length, the bar being of dimensions 1/4 by
1 by 8 inches.
[0034] The wax bar was withdrawn, sprinkled with a conventional
zircon stucco material, and allowed to dry.
[0035] The dried bar was then dipped into a second slurry
comprising 72.25 gallons (727 lbs. of liquid formulation) of Nalco
1130 colloidal silica with biocide and fungicides or Nalco 6011;
10.5 gallons (87.3 lbs.) of a binding polymer conventionally used
with Gray Matter.TM. silica formulation; 17.4 gallons of deionized
water; 38 bags (2090 lbs.) of the commercial Gray Matter.TM. silica
formulation; and 2.5 liters of anti-foam agent 2305, to provide a
slurry having a viscosity of 11-13 seconds as measured on a No. 5
signature Zahn cup.
[0036] Following this, a finely divided stucco material
(30.times.50 Mesh) of this invention, comprising about 70 weight
percent of silica and 30 weight percent of calcium carbonate, was
sprinkled on the bar which was coated with the second slurry
material, and allowed to dry.
[0037] This step of application of the second slurry, and
sprinkling with the silica-calcium carbonate stucco material, was
repeated three more times, with drying in between. Following this a
final application of the second slurry was provided by dipping of
the bar, followed by drying.
[0038] Then, the edges of each bar were removed with a belt sander,
providing two, spaced ceramic shell wall plates, separated by the
wax bar. The two remaining plates were separated from the wax, and
fired at 1000.degree. C. for two hours. The shell wall plates were
tested for modulus of rupture with a three point loading apparatus,
applying pressure in one direction at the two ends of the ceramic
face bar, and at the other direction in the middle of the ceramic
face bar, to obtain a modulus of rupture on breakage. Such wall
plates were tested with and without hydration, and also tested
against similar shell wall plates in which the stucco was made
primarily of silica, and was free of calcium carbonate.
[0039] Upon exposure to water spray at a wall plate bar temperature
of 25.degree. C., the plates that contained calcium formed cracks
within one hour, while the identical control shell wall plates not
exposed to water spray did not crack. However, merely by standing
in the atmosphere, the control shell wall plates that contained
calcium formed cracks within four hours.
[0040] When a water spray was applied to the shell wall plates
containing calcium while the plates were at a temperature of about
100.degree. C., cracks formed within four minutes, while the
similarly heated control shell wall plates having no calcium did
not exhibit any cracks under those conditions for a period of more
than four hours.
[0041] When a water spray was applied to the shell wall plates
containing calcium when the plates were the temperature of
200.degree. C., cracks formed in 11/2 minutes, while the control
shell wall plates, so heated, and free of calcium, did not exhibit
cracks over a period in excess of four hours.
[0042] From this, it can be seen that shell wall plates that
contain calcium carbonate are subject to weakening by hydration,
particularly when the shell wall plate is hydrated at an elevated
temperature. These shell wall plates are similar to portions of an
actual shell wall, and in the long term experience of the industry,
are good indicators of the behavior of actual investment casting
shells.
EXAMPLE 2
[0043] Sets of investment casting shells were prepared and used to
cast articles of aluminum using a conventional silica and zircon
slurry formulation. The investment casting shells used in this
invention contained 30 weight percent calcium carbonate in a
30.times.50 Mesh stucco 20 formulation (the remainder being
substantially silica). The control investment castings shells were
made identically to the casting shells containing calcium, except
that the control shells replaced the calcium carbonate with more
silica in the stucco.
[0044] The aluminum investment castings were made with six slurry
coatings, interspersed with five additions of stucco, in a manner
generally similar to that of Example 1. After firing of the shells
and investment casting, two shells containing aluminum castings
with different geometries were subjected to water blast to remove
the castings. They were also sprayed with water after pouring of
the mold, and prior to the water blast, while the temperature
remained elevated, to accelerate the hydration reaction.
[0045] The time involved to break away the shell from the first
aluminum parts, using a water blast system, was about 510 seconds
for the shells that contained calcium, and about 700 seconds for
the control shells that were free of calcium.
[0046] The time of water blasting required to fully remove the
shells from the second aluminum parts by means of a water blast
system was about 180 seconds for the shells that contained calcium,
and about 260 seconds for the shells that were free of calcium.
EXAMPLE 3
[0047] Bars for testing modulus of rupture were made in a manner
similar to claim 1 by repeatedly dipping a wax bar into a slurry,
the first five of such dips being followed by application before
drying of the previously described stucco mixture comprising 70
weight percent of finely divided fused silica and 30 weight percent
of calcium carbonate, the mixture being 30.times.50 Mesh. The
slurry formulation for each of the six dips comprised a mixture of
72 weight percent of Nalco Gray Matter.TM. silica slurry material;
0.16% Nalco 2305 antifoam agent; 0.05 weight percent Nalco 8815
wetting agent; and 27.79 weight percent of binder, which, in turn,
consisted of 70.2 weight percent Nalco 1130 colloidal silica; 20.8
weight percent of deionized water; and 9.0 weight percent of Nalco
6300 binding polymer. The resulting material was controlled to a
viscosity of about 18 seconds as indicated on a Guardco/ISO Mini
Dip viscosity cup with a 6 mm orifice, by the addition of Nalco
Gray Matter.TM. material as necessary. Six sequential dips were
provided, with the first five dips receiving a sprinkling of the
above-described stucco material and the sixth dip comprising a
sealing dip. The bars were allowed to dry between dips.
[0048] Then, a pair of faces of the bar were separated from the wax
as described in Example 1, to provide a pair of shell wall plates.
The shell wall plates were fired for 102 minutes at 1832.degree. F.
The shell wall plates were then exposed as they cooled to varying
humidity rates for 2-4 hours, and each was then tested for modulus
of rupture. This was compared to control shell wall plates which
were made in similar manner, but the calcium present in every layer
was replaced with more finely divided silica.
[0049] The results are as indicated in FIG. 1. The letters A
through D refer to the shell wall plates that contain calcium,
while "control (any RH)" refers to control bars in which the
calcium carbonate was replaced with finely divided silica, and for
which the humidity had no effect on the strength. The percentages
following each of letters A-D refer to the relative humidity of the
air in which the cooling bars resided for 2-4 hours, as identified
by the graph bars of varying shades, which shades indicate the time
of exposure to the humid air.
[0050] The various vertical lines 10 comprise error bars for the
data.
[0051] From these data, it can be seen that the respective shell
wall plates that contain calcium are significantly sensitive to
humid air, with the sensitivity tending to grow with the relative
humidity of the air to which they are exposed. For example, an
exposure of 4 hours to air of 90% relative humidity causes the
shell wall plate material almost disintegrate completely.
[0052] The above has been offered for illustrative purposes only,
and is not intended to limit the scope of the invention, which is
as defined in the claims below:
* * * * *