Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a binder composition for shell molds,
and more particularly to a process for the preparation of shell molds
for the casting of metals.
Precision casting of metal and other types of material in the
molten state is used in many industries and, generally, such castings are
made in expendable molds. There are three general types of processes
for making the expendable molds and these may be classified as the "lost
wax", the single investment and the double investment processes. These
all have one thing in common; they are one-use molds in which the mold
is generally destroyed in removing the casting therefrom. To provide an
economical process, a master mold or pattern is initially prepared from
which refractory molds are made by one of the above processes.
In the manufacture of precision castings by investment shell
casting techniques, disposable patterns are made from waxes, plastic
materials, frozen mercury -and other materials which readily may be
removed from the mold. The investment cycle consists of m~king the
pattern by injecting the pattern material into the die and gating the
pattern to a central sprue to form a pattern cluster.
Generally, the pattern cluster is dipped into an agitated
slurry of the coating composition, drained, stuccoed while still wet ~
with particulate mold material and aried. The dipping, draining, stucco- ;
ing and drying sequence is repeated he desired number of times, depen-
ding on the thickness and strength of the shell mold desired.
Thereafter, the disposable pattern is removed by methods such
as, for example, melting or solvent treatment and the mold cured by fir-
ing at a temperature sufficient to remove the volatiles and provide
adequate bonding. The molds are then heated and filled with molten
metal and after cooling, the castings are removed from the sprue and
finished in the usual manner.
The preparation of shell molds has been described in United
States Patent No. 3,270,382 issued Septe~ber 6, 1966
to Emblem et al in which two different coatings are applied alternately
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to an expendable pattern. One coating consists of a gel-~orming acid hy-
droly2ed alkyl silicate solution and the other coating consists of a gel-
forming basic silicate ester. m e acid hydrolyzed solution and the basic
silicate est~r o~ the applied coatings each act upon the other so as to
cause gelation of the other.
In United States Patent No. 3,079,656 issued ~arch 5, 1963 to
E~blem et al, the patentees describe the preparations of aminosilicates
by an interchange reaction bet~^7een an alkyl silicate and an aminoalcohol,
while removing the alcohol produced as the result of tne interchange reac-
tion. The aminosilicates may be co~bined with a reEractory material and
applied to an expendable pattern. The coating is gelled by the addition of
water. Likewise, amLnoaIk~lsilicates have heen disclosed as binders in
United States Patent ~o. 3,112,538 issued December 3, 1963 to Emblem et al.
The binder is combined with fine refractory powders to form a slurry which
is ap~lied to a wax pattern. Each coating is allowed partially to harden
before the next is applied. me coated pattern is then hardened for 24
hours before t~e wax pattern is removed. '~ -
Binders containing a mixture of an alkyl silicate and an aminoalkyl
silicate have been described in United S-tates Patent ~o. 3,329,520 issued
July 4, 1967 to Emblem et al. These binders are combined wlth a refractory
material and water and then cast into the desired s'nape.
A paint composition containing a mixture of organlc silicates and
an amine having a pKa value above 7.8 is described in Australian Patent
.~o. 163,467. However, wh~n the composition contains an amine such as, for
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example, monoethanolamine, the mixture must age for at least 48 hours and
up to fourteen days before it can be used as a protective coating on said
cores.
It is an object of one as~ect of this invention to provide a pro-
cess for preparing shell molds.
An object of another aspect of this invention is to provide a pro-
cess for preparing shell molds which utilize a single binder com~osition.
An object of another aspect of this invention is.to provide a pro-
cess for preparing shell molds ~ich may be used after air drying for from
1 to 3 hours.
~ n object of a further aspect of this invention is to provide a
binder composition which does not reouire the addition of additional water
to cause gelation thereof.
An object of a still fur~her aspect of this invention is to provide
a binder com~osition ~nich does not require removing the alcohol resul~ing
from the esterification of an organosilicate and an aminoalcohol.
By one broad aspect, then, a coating composition is provided for
the preparation of shell molds cc~prising a refractory material and a binder P
composition which is substantially free of water, the binder composition
comprising a silicate ester, an ammoalcohol of the formula
HOR~R')2
in which R is a divalent hydrocar~on radical having from 1 to 6 carbon atoms,
Rl is selected from the group consisting of hydrogen, an alkyl radical of
the formula -ROH in which R is the same as above, in which the mol ratio of
anuno-alcohol to silicate ester is from 1:1 to 10:1, and an i.
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organic solvent, thereby to form a binder composition naving an SiO2 content
of from 10 to 35 percent by weight based on the weiyht of the binder.
By a preferred variant, the silicate ester comprises a prehydro-
lyzed alkyl orthosilicate in which alk;yl group has from 1 to 4 carbon atoms.
By oth~r variants, the prehydrolyzed alkyl orthosilicate is
ethylpolysilicate or i5 silicate "40".
By yet oth~r variants, the aminoalcohol may ~e monoetnanolamlne or
triet7nanola~ine.
By another variant, the composition may include, as additional
additives, at least one of boron compounds and organic acids.
By one variant, t7ne boron compound is boric acid or an alhyl bor
ate in which the alkyl ~roup contains 1 to 8 carbon aton~7.
By another variant, tne acid is a mo.~car~oxylic acid haviny from -
1 to 10 carbon atoms.
By another aspeot of this invention, a process is provided for
preparin~ a sllell mold which comprises: coating an expendable pattern with
a oQmposition containing a binder which a,$ ~ubsta7ltially Eree of ~ater and
refractory material, in which the ratio of refractory material to binder
is from about 4 to 50 parts of refractory naterial per part of binder SiO2,
the binder ccmprisi~g a silicate ester, an a7ninoalconol of t7ne formwla
HO~N (R I ) 2
in which R is a dival~nt h ~ rbon radical having from 1 bD 6 carbon atQ~s~
R' is selected from the group oonæi~ting of hydrogen, an alkyl radical and
a radical o the formula -R~H in which R is the s~mei as above, in
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which the mol ratio of aminoalcohol to silicate ester is frorn 1:1 to 10:1
and sufficient organic solvent to provide an SiO2 content of from 10 to 35
percent by weight based on the weight of the binder; stuccoing the coated
pa~tern, air drying; and repeating the above sequence of steps until a coat-
ing of desired thickness is achieved.
By variants thereof, the si]icate ester may be a prehydrolyzed
aIkyl orthosilicate in which the alkyl group has from 1 to 4 carbon atoms
or the coating composition may also contain from 0.5 to 10 percent by ~7eight
based on the ~leight of the binder cornposition of an alkyl borate.
By yet another aspect, the process includes the step of applying
a seal coat, said seal coat comprising a composition containing a hydrolyzed
silicate ester, a refractory material an~ an organic solvent.
m e process of an aspect of this inventlon constitutes an improve-
ment in investment casting over that disclosed in the art. For e~ample,
the binder oQmposition of an aspec~ of this invention does not require an
aging period, i.e., the composition is ready to use substantially immediately
after muxing the lngredients. rloreover, the resulting shell is ready for r
use after air drying for from one to three .~ours. Also, the binder oom-
position of an aspect of this invention gels in the presence of atmosp.~eric
moisture and does not require adding additional water to the binder com~
position. In addition, the process of an aspect of this invention utilizes
a single binder composition to prepare shell molds, whereas many of the shell
molds prepared heretofore required at least two different types of binder
compositions.
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ccording to one varian-t and e~bodiment of this invention, previ-
ously claimed and e~ched disposable patterns are coated with a binder com~
position containing a refractory material. s-tuccoing with a coarse refractory
material and then dried to set the coating material. The sequence of steps
is repeated until the desired eoating tnickness is achieved. The coated
pattern thus formed may be sealed with a backup coating containing a hy-
drolyzed silicate ester, a solvent and a refractory material, if desired.
The sllicate esters found to be useful in providing compositions r`
of aspects~f the present invention are tetraethyl orthosilicate, "condensed"
ethyl silicate and ethyl silicate "40". "Condensed" ethyl silicate is
tetraethyl orthosilicate containing 10 percent polysilicates and is the un-
distilled reaction product of silicon tetrachloride and 200-proof ethanol
having a nominal composition of from 2 to 5 percent ethanol, 90 to 95 per-
cent orthosilicate and from 5 to 10 percent polyethoxy polysilicates.
Ethyl silicate "40" is a mixture of polysilicates averaging 5 silicon atoms
per m~lecule having a siliea content of 40 percent~ It is formed by reaet-
ing silicon tetrachloride ~ith ethanol containing some water under conditions r
such that a silica content of 40 percent is achieved. Other silicate esters
which may be ~mployed are those that
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are obtained from the hydrolysis and condensation of ethyl silicate "40".
Aminoalcohols which ma~ be employed in the binder composition
of an aspect of this invention may be represented by the formula
HORN(R')2
where R is a divalent hydrocarbon radical having from 1 to 6 carbon
atoms, R' is hydrogen or an alkyl group having from 1 to 6 carbon atoms
or a group of the formula -ROH in which R is the same as above.
Examples of suitable aminoalcohols are mono-, di-, or triethanolamine,
monoisopropanolamine, 2-amino-butan-1-ol, 3-amino-propan-1-ol, 2-dimethyl-
amino-2-methylpropan-1-ol and the like.
Any organic solvent which is a mutual solvent for the silicate
ester and the aminoalcohol may be employed. It is preferred that the
solvent have a boiling point below 250~C. and more preferably below 150C.
Examples of suitable organic solvents are aliphatic alcohols, such as,
for example, methanol, ethanol, isopropanol, butanol; ketones, such as,
for example, acetone, methyl ethyl ketone and aromatic hydroca~bons
such as, for example, benzene, toluene and the like. Polyethers which
may be employed as solvents in the binder composition of an aspect of
this invention are glycol ethers such as, for example, monoalkylene
glycol monoalkyl ethers, dialkylene glycol monoalkyl ethers 3 dialkylene
glycol dialkyl ethers and monoalkylene glycol dialkyl ethers. Examples
of preferred glycol ethers are ethylene glycol monomethyl ether,
ethylene glycol monoethyl ether, ethylene glycol monobutyl ether,
ethylene glycol dibutyl ether, ethylene glycol monohexyl ether, ethylene
glycol monophenyl ether, and esters of the ethylene glycols such as, for
example, ethylene glycol monobutyl ether acetate, ethylene glycol mono-
methyl ether acetate and the like.
The amount of aminoalcohol employed in the binder composition
of an aspect of this invention is not critical. It can be present in a
mol ratio of aminoalcohol to ethyl silicate of from 1:1 to lO:l and more
preferably from 3:1 to 8:1.
Generally the amount of silicate esters employed in the binder
composition of an aspect of this invention is sufficient to provide a
binder having an SiO2 content of the order from 10 to 35 percent and
more preferably from 15 to 30 percent by weight bas d on the weight of
the binder composition.
Refractory materials which may be employed in the binder com-
position of an aspect of this invention are many and varied. Generally,
well-known refractories such as, for example, the oxides and silicates -- -
of silicon, alumina, zirconium, zinc, magnesium, chromium and titanium
may be used. These include the acidic minerals, the neutral or amphot-
10 eric minerals, especially those whose primary elements are found in
Group II of the Periodic Table, the gel acid minerals, e.g., allophane
and the basic minerals, e.g., zirkelite. Also, there should be mentioned
the many synthetic materials including the zeolites, molecular sieves,
ion-exchange resins and similar substances. Apart from the foregoing,
zircon, grog, calcined clay, silica flour, sand anf fused silica may be
used providing compositions of aspects of t~is invention. Examples of
acidic, weakly acidic or neutral amphoteric minerals are fused silica,
natural silica, zircon, stillimanite, zirconite, whicll is a mixture of
silicates and oxides of zircon; mullite which is a high refractory
20 aluminum silicate, clays, magnesia9 and other metal oxides and silicates.
These refractory materials may be employed alone or in com-
bination depending on the particular casting operation contemplated.
Also, the particle size of the refractory material is selected depending
on the specific purpose for which the mold is being fabricated. In some
cases, it is desirable to employ a mixture containing both relatively
coarse and fine particles. G~nerally, a coarser refractory material is
used in the stuccoing step than is used in the initial coating composi-
tions, thereby providing a bond betweén one shell coat and the succes-
sive coat.
A shell mold conforming to an aspect of this invention is pro-
duced without the use of any other binder than the one described as an
aspect of this invention. However, where a seal coat or backup coat is
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desired a bin~er composition containing a refractory material and sili-
cate esters such as, for example, the prehydrolyzed type commercially
available or those prepared by the acid hydrolysis of any of the commer-
cially available forms of ethyl silicate, includinE monomeric orthosili-
cate, "condensed" ethyl silicate or ethyl silicate "40" may be employed.
To prevent premature gelation of the seal coat composition, a
small quantity of acid having an acidity greater than that of the
hydrolyzed ethyl silicate or an acidic buffering agent can be added to
the binder solution. The p~ oE the seal coat composition should be
maintained in the range from 1 to 3.2. Suitable acids and buffering
agents include hydrochloric acid, phosphoric acid, sulfuric acid, and
ammonium nitrate. The quantity of acid or buffering agent employed
will vary depending upon the acid or buffering agent used and the
refractory aterial present in the composition.
The same refraGtory materials which were employed in the prior
- coating composition may be used in the seal coat. Generally, these
refractory materials are oxides and silicates of silicon, alumina,
zirconium, zinc, tin, magnesium, chromium, titanium and the like. ~ther
refractory materials such as, for example, grog, calcined clay, silica
flour, sand or fused silica may also be used in the seal coat composi-
tion.
These refractory materials may be employed ~everally or in any
combination deemed desirable depending on the particular casting opera-
tion contemplated.
The same solvents which were used in the initial coating com-
position described above may be used in the seal coat. Examples of
suitable solvents are the alcohols, ketones and polyethers such as, for
example, the monoalkylene glycols monoalkyl ethers, dialkylene glycols
monoalkyl ethers, monoalkylene glycols dialkyl ethers and dialkylene
glycols dialkyl ethers.
The amount of binder employed in the coating composition is
substantially determined by the particle size of the refractory materials
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since the finer the refractory material the more readily it absorbs the
binder 9 hence the more binder required. In some cases where the
refractory particles are coarse, the amount of binder required may be
such that for every 100 parts refract:ory material there may be used only
that amount of binder necessary to provide 1 part of binder SiO2. Con-
versely, where the particles of refractory material are extremely fine
it may be necessary to use with each 4 parts of refractory material that
amount of binder necessary to provide 1 part of binder SiO2. In other
words, the ratio of refractory material to binder may range from 4 to
100 parts refractory material per part of binder SiO2.
In general, the seal coat composition is applied to the pre-
viously dried coating and then dried in the presence of atmospheric
moisture. In the event it is desired to accelerate the gelling of the
seal coat,` then a basic material su~h as, for example, ammonia or ~ -
organic amines such as, for example, aliphatic, aromatic and cyclic
amines having up to 10 carbon atoms may be employed. Examples of suit-
able amlnes are methylamine, mono-, di- and triethylamines, propylamine,
n-butylamine, cyclohexylamine, hydrazine, piperidine, and the like.
After the pattern has been covered with the desired number of
coats, it is dried at a temperature which may vary from 25C. up to the
fluidizing temperature of the disposable pattern. The temperature may
then be increased in a nonoxidizing atmosphere to fluidize and remove
the disposable pattern therefrom. The resulting shell mold is subse-
quently fired by exposure to a temperature of from 500 to 3000~C. in an
oxidizing atmosphere for from 1 to 12 hours.
Various aspects of the invention are illustrated by the follow-
ing examples in which all parts are by weight unless otherwise specified.
EXAMPLE 1
A wax tree pattern is sprued and gated to conform to the
requirements of the metal cast and casting desired. The tree is momen-
tarily etched in a 75 percent trichloroethylene - 25 percent isopropyl
alcohol mixture and the excess removed with isopropyl alcohol. The
.
p~ttern is allowed to dry before dipping into a coating composition
prepared in the following manner:
Binder Composition
Ingredients Parts
Ethyl silicate (condensed) ~ 416
Ethanol 540
Ethanolamine 244
The binder composition is prepared by adding with agitation
ethanolamine to a solution containing "condensed" ethyl qilicate and
ethanol at a temperature of from 40 to 45C. The solution i8 stirred
for one hour at this temperature, cooled to 30C. and filtered.
Approximately 300 parts of the binder composition prepared
above is mixed with 200 parts of ziroon (325 mesh) and 400 parts of
fused silica (available from Ransom and Randolph Company and identified
by the Trade Mark Rancosil #l) to form a slurry. The wax pattern is
coated with the slurry, stuccoed with a fused silica sand (available
from Ransom & Randolph Co~pany and identified by the Trade Mark
Rancosil A) and air dried for 15 minutes. The sequence of dipping and
stuccoing is repeated two times. The precoated pattern is then dipped
into a composition containing 300 parts of the above binder composition,
100 partsiof zircon (200 mesh) and 300 parts of fused silica (known by
the Trade Mark of Rancosil #2)~ stuccoed with a fused silica sand (known
by the Trade Mark of Rancosil B), and air dried for 15 minutes. The
sequence of dipping and stuccoing is repeated four times. The final
coat is dried for from 1 to 3 hours, dewaxed at a temperature of 200C.
and then fired at a temperature of 1000C. A shell mold free of imper-
fections is obtained.
EXAMPLE 2
The procedure of Example 1 is repeated except that 122 parts of
ethanolamine is employed in the preparation of the binder composition.
A shell mold having considerable strength and free of lmperfections is -
obtained.
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EXAMPLE 3
The procedure of Example l is repeated except that 416 parts
of ethyl silicate "40" is substituted for the "condensed" ethyl silicate.
Again a suitable shell mold free of imperfections is obtairted.
EXAMPLE 4
The procedure of Example l is repeated except that ethylene
glycol monoethyl ether is substituted for ethanol. A shell mold having
the desired strength is obtained.
E~YAMPLE S ~
A shell mold is prepared in accordance with the procedure of ~--
Example l except that the precoated pattern is coated with a backup coat ~-
consisting of the following ingrediénts.
Ingredients Parts by Weight _
Water ~ 0.5
Silbond H~4 (the Trade Mark for a 24.0 ..
partially hydrolyzed silicate
binder available from Stauffer
Chemical Company)
- Isopropyl alcohol 24.0
Fused silica flour (fine grain, 148.0
known by the Trade Mark of
Nalcast PlW available from
Nalco Chemical Company)
Water is added to the Silbond H-4 and the mixture allowed to
stand overnight before the alcohol and fused silica flour are added.
Approximately l to 3 milliliters of sulfuric acid is added for each
gallon of seal coat slurry to neutralize the basic ingredients.
The precoated pattern of Example l is dipped into the slurry
described above, excess slurry is removed and then stuccoed with a coarse
fused silica flour tknown by the Trade Mark of Nalcast S-2). The dipping
and stuccoing sequence is repeated four times. The coated pattern is
then dried o~Ternight and the wax removed by means of a high-pres~ure
steam autoclave. The resulting shell is fired at a temperature o:f 1000C.
and while still hot, molten stainless steel is poured into the cast.
The cast is then cooled and the shell is broken off by vibration. The
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castin~ showed no appreciable scaling or imperfections of any kind.
Other additives which may be incorporated in the binder com-
position of aspects of this invention are boron compounds and organic
acids. Examples of suitable boron compounds are boric acid and alkyl
borates in which each of the alkyl groups contairs from 1 to 8 carbon
atoms. The term "borate" includes any alkanol ester of any boric acid.
Suitable examples of alkyl borates ar~e trimethyl borate, triethyl borate,
tripropyl borate, triisobutyl borate, trimethoxy boroxine, tri-n-butoxy
boroxine, trihexoxy boroxine and the like. The borate or ~orate mixtures
can be added in pure form or in solvent solution as desired.
Suitable examples of organic acids which may be added to the
binder composition for preparing shell molds are organic monocarboxylic
acids having from 1 to 10 carbon atoms, such as, for example, formic
acid, ace~ic acid, propanoic acid, butanoic acid, hexanoic acid, octanoic
acid, decanoic acid, 2-ethylbutyric acid, 2-ethylpentanoic acid, f
~ 2-ethylhexoic acid and the like.
The amount of boron compound that may be added to the binder
composition ranges from 0.5 to 10 percent and more preferably from 1 to
5 percent by weight based on the weight of the binder composition. The
amount of organic acid that may be added ranges from 1 to 10 percent by
weight and more preferably from 1.5 to 5 percent based on the weight of
the binder composition.
The addition of the boron compound and the organic acid
improves the stability of the binder composition, i.e., these additives
improve the shelf life and the pot life of the resultlng coating composi-
tion O
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