Note: Descriptions are shown in the official language in which they were submitted.
2~~~3~~ _
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METHOD OF EXPENDABLE PATTERN CASTING
USING SAND WITH SPECIFIC THERMAL PROPERTIES
Backqround of the Invention
Expendable Pattern casting, also known as lost
foam casting, is a known casting technique in which a
pattern formed of an polymeric foam material, such as
polystyrene or polymethylmethacrylate, is supported in a
flask and surrounded by an unbonded particulate material,
such as silica sand. When the molten metal contacts the
20 pattern, the foam material decomposes with the products
of decomposition passing into the interstices of the sand
while the molten metal replaces the void formed by the
expended foam material to produce a cast part which is
identical in configuration to the pattern.
In the conventional expendable pattern casting
process, the sand which surrounds the pattern and fills
the cavities in the pattern is unbonded and free flowing
and this differs from traditional sand casting processes,
wherein the sand is utilized with various types of bind-
ers. However, after compaction, the unbonded sand
density is generally higher than the density of molds
made with bonded sand, and therefore the rigidity or
stiffness of compacted unbonded sand is not deficient
relative to bonded sand molds.
Traditionally, silica sand has been used
exclusively as the molding material in expendable pattern
casting because it is readily available and inexpensive.
A conventional expendable pattern casting process is only
capable of matching the precision of green sand casting
and has riot been considered a precision sand casting
process. This lack of precision for a process that uses
metal molds to make the foam patterns, is a drawback of
the process.
Aluminum silicon alloys have been cast utiliz-
ing expendable pattern casting techniques as disclosed in
U.S. Patent No. 4,966,220. Aluminum silicon alloys
containing less than about 11.60 by weight of silicon are
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referred to as hypoeutectic alloys and the unmodified
alloys have a microstructure consisting of primary alumi-
num dendrites, with a eutectic composed of acicular
silicon in an aluminum matrix. Hypoeutectic aluminum
silicon alloys have seen extensive use in the past but
lack wear resistance.
Hypereutectic aluminum silicon alloy, those
containing more than about 11.6% silicon, contain primary
silicon crystals which are precipitated as the alloy is
cooled between the liquidus temperature and the eutectic
temperature. nue to the high hardness of the precipitat-
ed primary silicon crystals, these alloys have good wear
resistance but are difficult to machine, a condition
which limits their use as casting alloys.
Normally, a solid phase in a '°liquid plus
solid" field has either a lower or higher density than
the liquid phase, but almost never the same density. If
the solid phase is less dense than the liquid phase,
floatation of the solid phase will result. On the other
hand, if the solid phase is more dense, a settling of the
solid phase will occur. In either case, an increased or
widened solidification range, which is a temperature
range over-which an allay will solidify, will increase
the time period for solidification and accentuate the
phase separation. With a hypereutectic aluminum-silicon
alloy, the silicon particles have a lesser density than
the liquid phase so that the floatation condition pre-
vails. Thus, as the solidification range is widened, the
tendency fox floatation of Large primary silicon parti-
cles increases, thus resulting in a less uniform distri-
bution of -silicon particles in the cast alloy. Converse-
ly, if the rate of cooling through the solidification
range is increased, the tendency for floatation of the
primary silicon particles is decreased resulting in a
more uniform distribution of smaller silicon particles in
the alloy. However, at sand casting cooling rates,
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improvements in wear resistance or machinability by using
different sand types, have not been recognized.
It is recognized in the casting art that using
a molding material that extracts heat more rapidly from
the molten metal and allows it to solidify at a faster
rate, yields a casting with superior mechanical proper-
ties. A cooling rate increase of three orders of
magnitude (i.e. a 1000 times increase) decreases the
dendritic arm spacing of the primary aluminum phase of
hypoeutectic aluminum-silicon alloys by one order of
magnitude (i.e. a factor of 10). This microstructure
change results in an increase in mechanical properties.
Thus, castings produced using metal molds, which extract
heat rapidly, generally exhibit superior mechanical
properties as opposed to castings produced by sand
casting or expendable pattern casting processes that
utilize sand as a molding material. However, when using
sand as a molding material, as in sand casting or
expendable pattern casting, daubling the cooling rate
(which is theoretically the most that can be expected
from the higher heat diffusivity abtainable with any sand
media), decreases the dendritic arm spacing of
hypoeutectic aluminum-silicon alloys by approximately 10%
and this reduction results in only a 5% increase in the
ultimate tensile strength. Thus sand casting properties
of hypoeutectic aluminum-silicon alloys are never listed
in the reference books by sand type.
One skilled in the metal casting art does not
expect the temperature of the sand to have a significant
influence on the dimensional size of castings produced by
any of the sand casting processes. The major reason for
this oversight is because, except for the expendable
pattern casting process which uses unbonded sand, sand
casting processes use bonded sand molds and these are
used at the semi-uncontrolled ambient temperatures seen
on 'the foundry floor. The economics of achieving
through-put in the foundry and the cost of carrying an
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unnecessarily high inventory of molds on the foundry
floor, dictate that the bonded sand molds be used in some
orderly just-in-time approach. As a result, it is not
the practice of foundries to heat or to cool the sand
molds in a separate "conditioning" or stabilizing area
and there has been no recognition that the temperature of
the sand mold has a significant effect on the dimensional
size or tolerances of the resulting castings that are
produced with the molds. '
Summary of the Invention
The invention is directed to a method of
expendable pattern casting utilizing a sand molding
material having specific physical properties to produce
castings having more precise dimensions or tolerances.
The invention has particular application to the casting
of engine blocks for internal combustion engines using
not only hypoeutectic aluminum-silicon alloys containing
about 4% to 11% by weight of silicon and hypereutectic
aluminum- silicon alloys containing from about 16% to 30%
by weight of silicon, but other aluminum-silicon alloys
between these silicon composition limits.
Tn the method of the invention, a polymeric
foam pattern is produced having a configuration corre-
sponding to the article to be cast. The foam pattern is
supported in a flask and an unbonded sand is fed into the
flask, surrounding the pattern and filling the cavities
in the pattern.
The sand has a heat diffusivity greater than
1500 J/Mz/K/s~h, a linear expansion from 0°C to 1600°C of
less than 1%, and a particle size in 'the range of 0.002
to 0.015 inch. Chromite sand, silicon carbide sand,
olivine sand, and carbon sand have properties falling
within these limits and are examples of sands which can
be utilized.
When the foam pattern is contacted by 'the
molten metal, the pattern will decompose and the products
of decomposition will be entrapped within the interstices
CA 02105361 1999-11-02
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of the unbonded sand while the metal will fill the space
initially occupied by the foam pattern, thereby producing
cast article which corresponds in configuration to the
a
foam pattern.
The thermal properties of the sand allow heat
to be extracted at a faster rate from the molten meta l
and coupled with the much smaller expansion of the sand
mold material, more precise castings are produced.
With the use of hypereutectic aluminum silicon
l0 alloys, the thermal properties of the sand and the faster
extraction of heat promotes enhanced under-cooling below
the liquidus of the molten metal and increased nucleation
of the primary silicon resulting in a smaller primary
silicon particle size in the cast article, thereby
improving the machinability of the casting.
As a further advantage, the use of the sand
with the above specified properties produces a more
uniform shrinkage of the cast metal on solidification,
resulting in a coefficient of variation of shrinkage of
less than 45%, as compared to a coefficient of variation
of shrinkage of about 50% when using silica sand. The
reduction in the coefficient produces a more precisely
dimensional casting.
Other advantages will appear in the course of the
following description.
Description of the Drawinas
The drawings illustrate the best mode presently
contemplated of carrying out the invention.
In the drawings:
Fig. 1 is a graph showing the linear expansion
of various sands with temperature; and
Fig. 2 is a graph showing the variation in
dimensions of a three cylinder engine block when using
silica sand at different temperatures.
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D~~eri~tienaof the Preferred Embodiment
The invention relates to a method of expendable
pattern casting utilizing unbonded sand having specific
thermal properties as a molding material.
In carrying out the invention, a polymeric foam
pattern is produced from a material such as polystyrene
or polymethylmethacrylate to provide a pattern having a
configuration corresponding to that of the article to be
cast. The foam pattern itself is produced by
20 conventional procedures.
As in conventional expendable foam casting, the
pattern can be coated with a porous ceramic material
which acts to prevent a metal/sand reaction and facili-
tates cleaning of the cast metal part. The ceramic
coating is normally applied by immersing the pattern in a
bath of ceramic wash, draining the excess wash from the
pattern and drying the wash to provide the porous ceramic
coating.
The process of the invention can be used with
any desired metal .or alloy and has particular application
in casting ferrous metals, such as cast iron or steel, or
aluminum-silicon alloys, either hypoeutectic or hyper-
eutectic aluminum-silicon alloys. In general, the
hypereutectic aluminum silicon alloys contain by weight
12o to 30% silicon, 0.4o to S.Oo magnesium, up to 0.30
manganese, up to 1.4o iron, up to 5.0% copper, and the
balance aluminum.
Specific examples of hypereutectic aluminum
silicon alloys to be used are as follows in weight per-
cent:
EXAMPLE 1
Silicon 16.90%
Iron 0.920
Copper 0.14%
Manganese 0.12%
Magnesium 0.41%
Aluminum 81.51%
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EXAMPLE 2
Silicon 20.10%
Iron 0.20%
Copper 0..33%
Manganese 0.18%
Magnesium o..71%
Aluminum 78.400
The hypoeutectic aluminum-silicon alloys
contain by weight less than 12% silicon, and one common
sand casting alloy contains from 6.5% to 7.5% by weight
of silicon, 0.25% to 0.450 by weight of magnesium, up to
0.6% iron, up to 0.2% copper, up to 0.25% titanium, up to
0.35% zinc, up to 0.350 manganese, and the balance
aluminum. Another common hypoeutectic aluminum--silicon
alloy that can be used in the invention contains from
5.5% to 6.5% by weight of silicon, from 3.0% to 4.0% by
weight of copper, from 0.1% to 0.5% by weight of
magnesium, up to 1.2% iron, up to 0.8% manganese, up to
0.5% nickel, up to 3.0% zinc, up to 0.25% titanium, and
the balance aluminum.
Specific examples of hypoeutectic~aluminum
silicon alloys to be used are as follows in weight
percent:
EXAMPLE 3
Silicon 7.10%
Magnesium 0.31%
Copper 0.05%
Titanium 0.05%
Zinc 0.100
Manganese 0.05%
Aluminum 92.210
210~3~~.
EXAMPLE 4
Silicon 6.210
Copper 3.150
Magnesium 0..32%
Iron 0.800
Manganese 0.520
Nickel 0.34%
Zinc 1.02%
Titanium 0.20%
Aluminum 87.35%
Traditionally, silica sand has been used as
the molding material in expendable pattern casting due to
the fact that silica sand is readily available and is
inexpensive. Through the development of the invention,
it has been discovered that the use of silica sand pres-
ents certain drawbacks when utilized in expendable patt-
ern casting procedures that were heretofore unrecognized,
and it has been further discovered that the unbonded sand
molding material should have certain physical properties,
not obtainable with silicon sand, in order to achieve
precision castings.
First, it has been found that the sand
temperature at the start of pouring in the expendable
pattern casting process has a significant effect on the
dimensional size of the casting produced in the process.
Secondly, it has been found that the co-
efficient of variation of shrinkage of the metal casting
is significantly improved if the sand molding material
has a linear expansion of less than 10
Thirdly, it has further been found that the
shrinkage value of the unbonded sand should nearly match
the' unconstrained shrinkage value of the cast metal,
unlike bonded sand castings with large cores, which
exhibit unpredictable lower shrinkage values, as compared
to the shrinkage value of the cast metal.
And fourthly, it has also been discovered
that when dealing with hypereutectic aluminum-silicon
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alloys, primary silicon particle size is primarily
affected by the initial cooling rate just below the
liquidus (for which sand type has an influence) rather
than by a faster average cooling rate through the entire
liquid plus solid solidification range and heavily
influenced by a high eutectic volume fraction.
The physical properties of sand, particularly
the thermal properties, greatly effect the precision of
casting when using expendable foam patterns. To provide
the improved precision in casting, the sand should have a
heat diffusivity greater than 1500 J/MZ/°K/s'~, a total
linear expansion from 0°C to 1600°C of less than 1%, and
a particle size in the range of 0.001 inch to 0.015 inch.
Chromite sand (FeCr204), silicon carbide sand, carbon
sand, and olivine sand (a solid solution of forsterite,
Mg2Si04, and fayalite, Fe2Si04) are examples of sands that
can be used in the process of the invention.
A comparison of the physical properties of
chromite sand, silicon carbide sand and silica sand are
shown in the following table.
TABLE I
Silicon
Silica Chromite Carbide
Sand Sand Sand
30
Thermal conductivity 0.90-0.61 1.09 3.25
(watts/m/°K)
Density (Kg/m3) 1500 2400 2000
Specific heat (J/Kg/°K) 1130-1172 963 840
Thermal diffusivity 0.360-0.512 0.472 2.0
(MZ/s x 10'6)
Heat diffusivity 1017-1258 1587 2340
(J/MZ/°KIs~~)
The thermal conductivity of a material is the
quantity of heat which flows per unit time through a unit
area of a mass of the material of unit thickness when
there is a difference of 1° in the temperatures across
opposite faces of the mass. The time rate of change of
r ~~.~~36~.
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the temperature, at any location is proportional to the
instantaneous slope of temperature gradient. The propor-
tionality constant is called the thermal diffusivity and
is defined as the thermal conductivity divided by the
volumetric heat capacity where the volumetric heat capac-
ity is the heat per unit volume necessary to raise the
temperature of the mass 1°.
The heat diffusivity, on 'the other hand, is a
measure of the rate at which the mold can absorb heat and
is the square root of the product of the thermal conduc
tivity, the density and the specific heat. As such, heat
diffusivity is directly related to solidification rate of
the molten metal.
It has been found that the linear expansion
of the sand with temperature is an important factor in
providing precise castings, and the linear expansion of
the sand should be less than 10 over a temperature range
of 0°C to 1600°C, and preferably less than 0.750, over a
temperature range of 0°C to 700°C. Figure 1 is a graph
showing the change in linear expansion of silica sand,
chromite sand and olivine sand with temperature. The
curve of silica sand shows a substantial increase in
expansion as the temperature of the silica sand approach-
es approximately 550°C. From the above graph, it is
noted that chromite and olivine do not undergo a similar
abrupt expansion as does the silica sand.
The importance of the linear expansion is
apparent when one compares the thermal diffusivity of a
casting metal, such as an aluminum alloy, with that of
sand. The thermal diffusivity of an aluminum alloy is
approximately 6.2 x 10'5 m'/s which is approximately 150
times greater than the thermal diffusivity of the sands
as shown in Table T above. This means that the average
distance through which heat flows in a given time is
approximately 12 times greater for the aluminum alloy
than for sand, resulting in a heat build up at the sand/
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metal interface which causes the sand mold cavity to
expand. Since the thermal expansion coefficient of
silica sand is approximately 4 times greater than that of
chromite sand, any temperature increase at the metal/sand
interface will cause the silica sand to expand substanti-
ally more than chromite sand and therefore will produce a
larger dimensional casting. Also, since the molten
metal/sand interface has moved outward before the start
of solidification, the calculated shrinkage value obtain-
ed on the larger casting will result in an apparent lower
(and unpredictable) shrinkage value for the solidified
metal.
As noted above, the heat diffusivity of the
sand is directly related to the solidification rate of
the molten metal. From the heat diffusivity data shown
in Table I above, it is seen that the use of chromite
sand should increase the solidification rate of the
metal, i.e. the time required to pass between the
liquidus and solidus temperatures, over that using silica
sand by approximately 26% to 56o due to the greater heat
diffusivity of the chromite sand. This improvement in
the solidification rate in itself may not be seen as a
worthwhile economic advantage but when considered with
the large expansion that occurs with silica sand at about
550°C, a substantial improvement in the precision of the
castings is achieved.
Furthermore, an unexpected enhanced nucleat-
ing phenomena is obtained when utilizing hypereutectic
aluminum silicon alloys in the process of the invention.
With hypereutectic aluminum silicon alloys, primary
silicon crystals are precipitated as the alloy is cooled
from solution temperature. When hypereutectic aluminum
silicon alloys are cast in the process of the invention
using the sand of the above-noted thermal properties,
heat is more readily extracted from the molten metal
(before heat saturation occurs at the molten metal/sand
interface) which contributes to enhanced undercooling
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below the liquidus temperature of the alloy which in turn
promotes increased nucleation of the primary silicon
resulting in a smaller silicon particle size in the cast
article. The reduction in particle size of the silicon
improves the machinability of the alloy making the cast
alloy more valuable for articles such as engine blocks.
From the data in Table I, and a comparison of
the heat diffusivity values, the use of chromire sand
produces about a 27o increase in the solidification rate,
20 as opposed to the use of silica sand. With hypoeutectic
aluminum-silicon alloys a 27% increase in the solidifica-
tion rate results in an insignificant improvement in
mechanical properties (i.e. decrease in the primary
aluminum dendrite arm spacing). However, with hyper-
eutectic aluminum-silicon alloys the primary silicon
particle size appears to be very sensitive to a marginal
increase of 27o in the solidificai:ion rate. The reason
for this different sensitivity is because different
fundamental mechanisms are operating. The mechanical
properties of hypoeutectic aluminum-silicon alloys are
controlled by the dendrite arm spacing of the primary
aluminum and this spacing is controlled by a growth
mechanism dictated by the solidification rate. The
silicon particle size of hypereutectic aluminum-silicon
alloys is controlled by a nucleation mechanism and 'this
in turn is controlled by the character of the under-
cooling immediately below the liquidus.
A consideration of the microstructure
difference between hypoeutectic and hypereutectic
aluminum-silicon allays is quite helpful in illustrating
how the heat of fusion is dissipated during solidifica-
tion in these alloys. At a temperature slightly above
the eutectic temperature, a hypoeutectic aluminum alloy
of Example 3 (microstructure: 40% primary aluminum, 60%
eutectic liquid) and a hypereutectic aluminum-silicon
alloy of Example 1 (microstructure: 10o primary silicon,
90o eutectic liquid) both have given up approximately an
~1~~351
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equal amount of heat to the sand molding material, since
the heat of fusion of silicon is 4.5 times that of
aluminum. However, as the remaining eutectic liquid
solidifies, the hypereutectic alloy gives up increment-
s ally 50% more heat on a volume basis (since it contains
50% more eutectic liquid) than the hypoeutectic alloy
(38% on an overall basis), which inherently slows the
solidification of the hypereutectic alloy compared to the
hypoeutectic alloy. The insight into obtaining a smaller
primary silicon particle size in the hypereutectic alloy
is to focus on the temperature range immediately below
the liquidus temperature where the nucleation phenomena
can be affected by an incremental faster cooling rate and
not t4 focus on the temperature where silicon rejection
is being accommodated by the growth of existing primary
silicon particles. Thus, the use of chromite sand in the
expendable pattern casting process, which can affect a
26-56% increase in the solidification rate (which shows a
parabolic dependence with time and is therefore more
effective in "early" time rather than in "late" time) has
a most significant effect on primary silicon particle
size, and this has heretofore not been recognized.
By comparison, die casting with large metal
molds that function as very large heat sinks, overwhelms
the nucleation at temperatures immediately blow the
liquidus temperature, as well as at temperatures immedi-
ately below the eutectic.temperature. As a result, the
die cast microstructure for a hypereutectic aluminum-
silicon alloy, even containing no phosphorous additions,
consists of a refined primary silicon as well as a
refined eutectic silicon.
The microstructure of an expendable pattern
cast hypereutectic aluminum-silicon alloy, by contrast,
does not contain refined eutectic silicon in the
microstructure because the cooling rate is far too slow.
In fact, primary silicon refinement in a hypereutectic
aluminum-silicon alloy requires phosphorous additions in
2~.053~~.
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all casting processes except a die casting process. In
Gruzleski and Closset, "The Treatment of Liquid
Aluminum-Silicon Alloys" (American Foundrymen's Society,
Inc., 1990) it is stated that "hypereutectic alloys such
as 390 are very difficult, if not impossible, to sand
cast. Even with phosphorous treatment, solidification
rates are so slow that unacceptably large primary phase
particles form and float to the upper surfaces of the
casting." Therefore, it has been believed that the
cooling rate has an effect on the primary silicon
particle size in a phosphorous treated alloy, but that
the cooling rate effect refers to the entire liquid-solid
range, as in die casting, and not just to the upper
portion of the liquid-solid range. It is believed that
the reason for this erroneous insight is that hyper-
eutectic aluminum-silicon alloys have not been considered
viable sand casting alloys and, therefore, sand casting
developments with hypereutectic aluminum-silicon alloys
have nct been investigated for commercial use, and
further primary silicon particle size has never been
studied, as a function of sand type, in any of the
various sand casting processes
The reduction in particle size of the silicon
crystals can be illustrated by a comparison of casting a
hypereutectic aluminum silicon alloy in an expendable
pattern process using silica sand as a molding material
as compared to using chromite sand. In this comparison,
the aluminum silicon alloy contained lB.Oo silicon, 0.69%
magnesium, 0.1% copper, and the balance aluminum. The
molten aluminum silicon alloy was poured at a temperature
of 704°C (1300°F) into a flask containing silica sand at
26.7°C (80°F) and into a second chromite sand flask at
26.7°C (80°F), both containing a polystyrene sprue with
three polystyrene foam patterns of a 60 horsepower 3
cylinder engine block connected to the sprue. Differenc-
es in the primary silicon particle size were measured in
the cast engine blocks with the two sand types. The
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average primary silicon particle size obtained by measur-
ing 849 silicon particles utilizing silica sand in the
casting was 30 microns with a coefficient of variation of
primary silicon particle size of 500. The average prim-
ary silicon particle size obtained by measuring 442
silicon particles produced using the chromite sand was
21.4 microns and a coefficient of variation in that
average of 37%. Thus, the use of silica sand gave an
average primary silicon particle size which was 39%
larger than that obtained through use of chromite sand
and the coefficient of variation of the particle size
using silica sand was substantially greater than that
obtained with the chromite sand. In general, the average
primary silicon particle size of the cast hypereutectic
aluminum-silicon alloy produced by the invention is less
than 30 microns and the coefficient of variation of
particle size is less than 500
This test evidences the unexpected reduction
in silicon particle size in a hypereutectic aluminum
silicon alloy that. is achieved when using the specified
sand in an expendable pattern casting process.
When casting an engine block for an internal
combustion engine, the pattern is formed with a plurality
of cylindrical bores which correspond to the cylinders in
the cast block. In the flask the sand nat only surrounds
the pattern, but also fills the bores thus providing sand
cores. During casting, the molten metal will shrink as
it solidifies. If the sand core does not "give" as the
metal solidifies and shrinks around it, stresses can be
set up in the casting and unpredictable diameters will be
obtained in the cylinder bores. Thus, the sand used as
the core should permit the core to follow the shrinkage
of the solidifying metal.
The following Table summarizes repeat
measurements of the average of twenty-five different
critical dimensions of a complex 60 ~iP, three-cylinder,
marine engine block using various sand molding materials
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in an expendable foam casting process and using the
aluminum-silicon alloy of Example 3, above. The results
show that when using chromite sand, silicon carbide sand,
or carbon sand the shrinkage of the alloy has nearly
matched the unconstricted contraction of the alloy as
reported. in the literature. This is quite surprising,
because complex engine blocks with large cores and
produced in a sand casting process using bonded sand,
generally exhibit smaller contraction results than the
unconstricted contraction of the alloy. The different
contraction results in the alloy, as shown in Table II,
are believed to be the result of different degrees of
constraint by the sand mold (and core) during cooling.
It is recognized that the hardness of ramming of the sand
and the percentage of binder in chemically bonded sands
significantly affects the contraction. Based on the
above, the unbonded sand in the expendable foam casting
process can be viewed as inherently offering less of a
constraint during cooling than bonded sand and, therefore
more sensitive to the phenomena of the expansion of the
mold fram molten metals of higher heat content and/or
from starting the casting process with heated sand. This
latter factor is clearly reflected by the shrinkage
values in the alloy of 0.00925 inch per inch and 0.007
inch per inch for 80°F and 160°F silicon sand,
respectively. The larger dimensioned cast engine blocks
resulting from the.use of heated silica sand simply
reflect the larger expansion of the sand mold as a result
of the higher sand temperature.
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TABLE II
Average Coefficient
Sand Sand Shrinkage of Variation
Type Temperature (in Lin) of Shrinkage
S7.liCd 160F 0.0070 600
SllICa 80F 0.0093 500
Chromite 80F 0.0119 35%
Silicon 80F 0.0110 37%
carbide
Spherical 80F 0.0106 360
Carbon sand
From the above table, it can be seen.that the
use of chromite sand, silicon carbide sand, and carbon
sand, resulted in a greater metal shrinkage rate in
inches per inch than when using silica sand thus enabling
an unbonded sand Core to more Closely follow the shrink-
age of the alloy.
Equally important is the fact that the use of
Chromite sand, silicon carbide sand and Carbon sand
produced a substantially lower coefficient of variation
of shrinkage in the metal casting, as compared to the use
of silica sand. This means that the shrinkage at the
various locations of measurement was more uniform and had
less variance than that measured when using silica sand.
Repeat measurements on a cast 250 HP, V-6, 3
liter, marine engine block gave similar results. The
ambient temperature, 80°F, silica sand yielded a shrink-
age value of 0.0094 inch per inch, and the ambient
temperature chromite sand yielded a shrinkage of 0.0118
inch per inch. In addition, the ambient temperature
silica sand gave a precision, reflected by the
coefficient of variation (of shrinkage), more than 500
less than that obtained through use of the Chromite sand.
These results further evidence that silica sand, as the
molding material, produces larger dimensional engine
blocks than the use of chromite sand. Moreover, the
precision obtained with silica sand is significantly less
~~.~~3~1
- 18 -
than the precision obtained with chromite sand. The test
results also indicate that the geometry differences
between a V-6 engine block and an in-line three--cylinder
block do not materially affect the shrinkage values
obtained for the two different sand types.
The particle size of the sand should be in
the range of 0.001 inch to 0.015 inch. An expendable
pattern casting process requires a relatively coarse sand
because of the sand permeability requirements and metal
fill time requirements which are interrelated. In
expendable pattern casting, the molten metal cannot
completely fill the sand mold cavity until the products
of foam decomposition have entered the interstices
between the sand grains.
Not only are the thermal properties of the
sand important in providing precision castings, but it
has also been found that the temperature of the sand
influences the casting. For example, in winter candi-
tions in the foundry, the sand temperature may be in the
range of 18.3°C (65°F) to 29.4°C (85°F). In
summer,
where the ambient temperature may be up to 32.2°C (90°F)
or higher, the sand temperature can be in the range of
29.4°C (85°F) to 40.5°C (105°F). With the higher
sand
temperature in summer, the castings will have a somewhat
larger dimension than castings produced in the winter
with the sand at a lower temperature. Therefore, to
compensate for this differential in dimensions in the
cast part, the size of the expendable foam patterns can
be adjusted. The dimension of the pattern can be changed
by aging the plastic beads before molding, or by aging
the molded parts after molding, or by selecting another
foam bead type. Thus, by proper aging or selection of
the beads, a larger pattern can be obtained which can be
used in the winter to compensate for the lower sand
temperature, thus resulting in cast parts which have the
same dimensions regardless of the temperature of the
sand.
- 19 -
Fig. 2 further illustrates the importance of
the sand temperature on the precision of casting. Fig. 2
is a curve showing average measurements of an engine
block dimension in inches as a function of the tempera-
s Lure of unbonded silica sand used in an expendable
pattern casting process. The engine block was cast from
a hypoeutectic aluminum-silicon alloy having the
composition of Example 3 above. As seen in Fig. 2, the
average engine block dimension when using sand at ambient
temperature of 80°F was 9.53 inches. As the sand
temperature was increased to 160°F, the average block
dimension also increased. to a value of about 9:59 inches,
or an increase of 0.06 inch.
While the above curve shows the difference in
dimensions obtained by using silica sand at various
tempertures, similar expansion results, although smaller
by a factor of approximately four, are obtained using
chromite sand, silicon carbide sand, or carbon sand, thus
indicating that sand temperature is a factor in obtaining
precisely dimensioned castings.
' The invention is based on the discovery that
more precise castings can be produced in an expendable
pattern casting process by utilizing sand having specific
thermal properties and cntrolling the sand temperature or
correlating the sand temperature with the pattern size.
When dealing with hypereutectic aluminum silicon alloys,
the invention provides a second and unexpected advantage
in that the particle size of the precipitated silicon
crystals is reduced which substantially improves the
30~ machinability of the alloy.
Various modes of carrying out the invention
are contemplated as being within the scope of the
following claims particularly pointing out and distinctly
claiming the subject matter which is regarded as the
invention.
