Chapter 14
EXPENDABLE-MOLD CASTING
PROCESSES
14.2 SANDCASTING
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Sand casting, by far the most popular of the casting
processes, uses ordinary sand as the primary mold
material.
The sand grains are mixed with small amounts of
other materials, such as clay and water, to improve
moldability and cohesive strength, and are then
packed around a pattern that has the shape of the
desired casting.
If the pattern must be removed before pouring, the
mold is usually made in two or more pieces.
Patterns and Pattern Materials
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The first step in making a sand casting is the design
and construction of a pattern.
This is a duplicate of the part to be cast, modified in
accordance with the requirements of the casting
process, metal being cast, and particular molding
technique that is being used.
The pattern material is determined primarily by the
number of castings to be made but is also influenced
by the size and shape of the casting, the desired
dimensional precision, and the molding process.
Patterns and Pattern Materials
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Wood patterns are relatively easy to make and are
frequently used when small quantities of castings are
required.
Wood, however, is not very dimensionally stable.
It may warp or swell with changes in humidity, and it
tends to wear with repeated use.
Metal patterns are more expensive but are more
dimensionally stable and more durable.
Hard plastics, such as urethanes, offer another
alternative, and are often preferred with processes
that use strong, organically bonded sands that tend
to stick to other pattern materials.
Types of Patterns
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Since many types of patterns are used in the foundry industry,
selection is usually based on the number of duplicate castings
required and the complexity of the part.
One-piece or solid patterns, such as the one shown in Figure
14-2, are the simplest and often the least expensive type to
make.
This type of pattern is essentially a duplicate of the part to be
cast, modified only by the various allowances discussed in
Chapter 13 and they are relatively cheap to construct, but the
molding process is usually slow.
One-piece patterns, therefore, are generally used when
the shape is relatively simple and the number of duplicate
castings is rather small.
Types of Patterns
Types of Patterns
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Split Patterns are used when moderate quantities of
duplicate castings are to be made.
The pattern is divided into two segments along a
single parting plane, which will correspond to the
parting plane of the mold.
Match-plate patterns, such as the one in Figure 14-5,
further simplify the process and can be coupled with
modem molding machines to produce large quantities
of duplicate castings.
The cope and drag segments of a split pattern are
permanently fastened to opposite sides of a wood or
metal match plate.
Types Of Patterns
Types Of Patterns
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When the product has protruding sections arranged
such that a one-piece or split pattern could not be
removed from the molding sand, a loose-piece
pattern can sometimes be developed.
Loose pieces are held to the primary segment of the
pattern by beveled grooves or pins.
After molding, the primary segment of the pattern is
removed by direct withdrawal. The hole that is
created then permits the remaining segments to be
moved in the direction necessary for their extraction.
In some loose-piece patterns, a single sliding pin is
used to hold all the segments in place.
Types Of Patterns
Sand Conditioning
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The sand used to make molds must be carefully prepared if it is
to provide satisfactory and uniform results. Ordinary silica
(SiOz), zircon, or olivine (forsterite and fayalite) sands are
compounded with additives to meet four requirements:
1. Refractoriness: the ability to withstand high temperatures
2. Cohesiveness: (also referred to as bond): the ability to retain a
given shape when packed into a mold
3. Permeability: the ability to permit gases to escape through it
4. Collapsibility: the ability to permit the metal to shrink after it
solidifies and ultimately to free the casting by disintegration of
the surrounding mold.
Sand Conditioning
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Refractoriness is provided by the basic nature of the
sand. Cohesiveness, bond, or strength is obtained by
coating the sand grains with clays, such as bentonite,
kaolinite, or illite, that become cohesive when
moistened.
Collapsibility is sometimes obtained by adding cereals
or other organic material, such as cellulose, that bum
out when they come in contact with the hot metal.
The combustion reduces both the volume and
strength of the restraining sand.
Permeability is a function of the size of the sand
particles, the amount and type of clay or bonding
agent, the moisture content, and the compacting
pressure.
Sand Testing
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Standard tests and procedures have been developed
to evaluate grain size, moisture content, clay content,
and compactability, as well as mold hardness,
permeability, and strength.
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Moisture content is usually determined by a special
device that measures the electrical conductivity of a
small sample of sand that is compressed between
two prongs.
Another method is to measure the weight lost from a
50-g sample after it has been subjected to a
temperature of about 230°F (l100C) for sufficient
time to drive off all the water.
Sand Testing
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Clay content can be determined by washing the clay
from a 50-g sample of molding sand in water that
contains sufficient sodium hydroxide to make it
alkaline.
Several cycles of agitation and washing may be
required to fully remove the clay. The remaining sand
is then dried and weighed to determine the amount
of clay in the original sample.
Permeability and strength tests are conducted on a
standard rammed specimen. A sufficient amount of
sand is placed into a 2-in.-diameter steel tube so that
after a 14-lb weight is dropped three times from a
height of 2 inch the final height of the specimen is
within 1/32 in of 2 in.
Sand Testing
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The hardness of the compacted sand can provide a
quick indication of mold strength and give additional
insight into the strength-permeability characteristics.
Compactability is determined by sifting sand into a
steel cylinder, leveling off the column, striking it
three times with a standard weight (as in the
permeability test), and then measuring the final
height.
The percent compactability is the change in height
divided by the original height, times 100%.
Sand Properties and related
Defects
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The characteristics of the sand granules can be very
influential in determining the properties of the
molding material. Round grains give good
permeability and minimize the amount of clay
required because of their low surface area.
Angular sands give better green strength because of
the mechanical interlocking of the grains.
Large grains provide good permeability and better
resistance to high temperature melting and
expansion, while fine-grained sands produce a better
surface finish on the final casting.
Uniform-size sands give good permeability, while a
wide distribution of sizes enhances surface finish.
Sand Properties and related
Defects
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When hot metal is poured into a silica sand mold, the sand
becomes hot, undergoes one or more phase transformations,
and has a substantial expansion in volume.
Because sand is a poor' thermal conductor, only the sand that is
adjacent to the mold cavity becomes hot and expands.
The remaining material stays fairly cool, does not expand, and
provides a high degree of mechanical restraint. Because of this
uneven heating, the sand at the surface of the mold cavity may
buckle or fold.
Castings having large, flat surfaces are more prone to sand
expansion defects since a considerable amount of expansion
must occur in a single, fixed direction.
Sand Properties and related
Defects
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Sand expansion defects can be minimized in a
number of ways. Certain sand geometries
permit the grains to slide over one another,
thereby relieving the expansion stresses.
Excess clay can be added to absorb the sand
expansion, or volatile additives, such as
cellulose, can be added to the mix.
As the sand becomes hot, the cellulose bums,
creating voids that can accommodate the
sand expansion.
Sand Properties and related
Defects
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Castings can also contain voids that form where the
molten metal is held back by trapped or evolved gas.
These are usually attributed to low sand permeability
and/or large amounts of gas evolution caused by
high moisture or excessive amounts of volatiles.
If adjustments to the mold composition are not
sufficient to eliminate the voids, vent passages may
have to be cut, a procedure that adds significantly to
the mold-making cost.
Sand Properties and related
Defects
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The molten metal can also penetrate between the
sand grains, causing the mold material to become
embedded in the surface of the casting.
Penetration can be the result of high pouring
temperatures (excess fluidity), high metal pressure,
or the use of high-permeability sands with coarse,
uniform particles.
Fine-grained materials, such as silica flour, can be
used to fill the voids, but this reduces permeability
and increases the likelihood of gas and expansion
defects.
Desirable Mold Properties
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Strength - to maintain shape and resist erosion
Permeability - to allow hot air and gases to pass
through voids in sand
Thermal stability - to resist cracking on contact with
molten metal
Collapsibility - ability to give way and allow casting to
shrink without cracking the casting
Reusability - can sand from broken mold be reused to
make other molds?
Green Sand, Dry Sand and
Skin Dried Molds
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In green-sand molding, the mold material is
composed of sand with a binder of clay, water, and
additives.
Tooling costs are low, and the entire process is quite
inexpensive. Almost any metal can be cast, and there
are few limits on the size, shape, weight, and
complexity of the products.
Design limitations are usually related to the rough
surface finish, poor dimensional accuracy, and the
need for subsequent machining.
Still other problems can be attributed to the low
strength of the mold material and the moisture that
is present in the binder.
Green Sand, Dry Sand and
Skin Dried Molds
Green Sand, Dry Sand and
Skin Dried Molds
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The problems of the green-sand process can be reduced by
heating the mold to a temperature of 300°F or higher, and
baking until most of the moisture is driven off.
This strengthens the mold and reduces the amount of
gases generated when the hot metal enters the cavity.
These dry-sand molds are not very popular, because of the
long times required for drying, the added cost of that
operation, and the availability of practical alternatives.
An attractive compromise is to produce a skin-dried mold,
drying only the sand that is adjacent to the mold cavity.
Torches are often used to perform the drying, and the
water is usually removed to a depth of about one-half inch.
Green Sand, Dry Sand and
Skin Dried Molds
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Molds used for the casting of steel are almost always
skin-dried, because the pouring temperatures are
significantly higher than those for cast iron.
These molds may also be given a high-silica wash
prior to drying to increase the refractoriness of the
surface, or the more-stable zircon sand can be used
as a facing.
Additional binders, such as molasses, linseed oil, or
corn flour, may be added to the facing sand to
provide additional strength to the skin-dried segment.
Sodium Silicate- CO2 Molding
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Molds (and cores) can also be made from a sand that
receives its strength from the addition of 3 to 4%
sodium silicate, a liquid inorganic binder that is also
known as water glass.
The sand can be mixed with the liquid sodium silicate
in a standard muller and can be packed into flasks by
any of the methods discussed previously.
It remains soft and moldable until it is exposed to a
flow of C02 gas, when it hardens in a matter of
seconds by the reaction.
Sodium Silicate- CO2 Molding
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The C02 gas is nontoxic and odorless, and no heating
is required to drive the reaction. The hardened
sands, however, have poor collapsibility, making
shakeout and core removal difficult.
Unlike most other sands, the heating that occurs as a
result of the pour makes the mold even stronger (a
phenomenon similar to the firing of a ceramic
material).
Additives that will burn out during the pour are often
used to enhance the collapsibility of sodium silicate
molds.
In addition, care must be taken to prevent the
carbon dioxide in the air from hardening the sand
before the mold-making process is complete.
No-Bake, Air-Set, or Chemically
Bonded Sands
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An alternative to the sodium silicate process involves
the use of organic resin binders that, cure by
chemical reactions that occur at room temperature.
Two or more binder components are mixed with the
sand just prior to the molding operation, and the
curing reactions begin immediately.
Because the mix is workable for only a short period
of time, the molds (or cores) must be made in a
reasonably rapid fashion.
After a few minutes to a few hours (depending on
the specific binder and curing agent), the sands
harden enough to be removed from the pattern and
are ready to pour.
No-Bake, Air-Set, or
Chemically Bonded Sands
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Various no-bake sand systems are available, with
selection being based on the metal being poured and
the specific sand performance characteristics that are
desired.
Each system is based on organic resin binders, curing
agents or catalysts, and various additives and
modifiers.
Like the sodium silicate molds, no-bake offers high
dimensional accuracy, good hot strength, and high
resistance to mold-related casting defects.
Patterns can incorporate thinner sections and deeper
draws. In contrast to the sodium silicate material,
however, the no-bake molds decompose readily after
the metal has solidified, providing excellent shakeout
characteristics.
Shell Molding
Many molds are now being made by the shellmolding process, which offers better surface finish
than can be obtained with ordinary sand molding,
better dimensional accuracy, and a higher production
rate with reduced labor requirements.
 The six steps involved in Shell Molding are as follows;
1. A mixture of sand and thermosetting plastic binder is
dumped onto a metal pattern that has been heated
to 300 to 450°F . It is allowed to stand for a few
minutes while the heat from the pattern partially
cures a layer of the sand-plastic mixture. This forms
a strong, solid-bonded region about 3.5 mm thick
adjacent to the pattern.
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Shell Molding
2. The pattern and sand mixture are then inverted. All of
the excess sand drops free, leaving only the layer of
partially cured material that adhered to the pattern.
3. The pattern and partially cured "shell" are then
placed in an oven for a few minutes to complete the
curing process.
4. The hardened shell is then stripped from the pattern.
5. Two or more shells are then clamped or glued
together to produce a mold.
6. The bonded shells are often placed in a pouring
jacket and surrounded with metal shot or sand to
provide extra support during the pour.
Shell Molding
Shell Molding
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Because the sand is compounded for almost no
shrinkage and a metal pattern is used, the shell has
excellent dimensional accuracy.
Shell-mold sand is finer than ordinary foundry sand
and, in combination with the plastic resin, produces a
very smooth shell and casting surface.
Cleaning, machining, and other finishing costs can be
reduced significantly.
In addition, the shell mold process offers a product
consistency that is superior to that of green-sand
casting.
Other Sand-Based Molding
Methods
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Over the years, various processes have been
proposed to overcome some of the limitations of the
more traditional methods.
While few have become commercially significant,
several are included here to illustrate the nature of
these efforts.
In one method, known as the V-process or vacuum
molding, a vacuum is used in place of a sand binder.
Figure 14-19 depicts the production sequence, which
begins by draping a thin sheet of plastic over a
special pattern, which is then drawn tightly to the
pattern surface by a pattern vacuum.
Other Sand-Based Molding
Methods
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A vacuum flask is then placed over the pattern, the
flask is filled with sand, a sprue and pouring cup are
formed, and a second sheet of plastic is placed over
the mold.
A second vacuum is then drawn on the flask itself,
compacting the sand and providing the necessary
strength and hardness.
The pattern vacuum is released, the pattern is
withdrawn, and the mold halves are assembled.
The mold is poured while maintaining a vacuum in
both the cope and drag segments of the flask
Other Sand-Based Molding
Methods
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Advantages of the vacuum process include the total
absence of moisture-related defects.
Since no binder is used, binder cost is eliminated and
the sand is completely reusable.
No fumes (binders burning up) are generated during
the pouring operation.
Shakeouts characteristics are exceptional; the mold
virtually collapses when the vacuum is released.
Unfortunately, the process is relatively slow because
of the additional steps and the time required to pull a
sufficient vacuum.
14.3 Cores and Core Making
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Casting processes are unique in their ability to
incorporate internal cavities -or reentrant sections
with relative ease.
To produce these features, however, it is often
necessary to use cores as part of the mold.
Figure 14-20 shows an example of a product that
could not be made by any process other than casting
with cores.
While these cores constitute an added cost, they do
much to expand the capabilities of the process, and
good design practice can often facilitate and simplify
their use.
14.3 Cores and Core Making
14.3 Cores and Core Making
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Consider the simple belt pulley shown schematically
in Figure 14-21. Various methods of fabrication are
suggested in the four sketches, beginning with the
casting of a solid form and the subsequent machining
of the through-hole for the drive shaft.
A large volume of metal would have to be removed
through a substantial amount of costly machining.
A more economical approach would be to make the
pulley with a cast-in hole of the approximate final
size.
14.3 Cores and Core Making
14.3 Cores and Core Making
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Figure 14-21b depicts an approach where each half
of the pattern includes a tapered hole, Which
receives the same green sand that is used for the
remainder of mold.
While these protruding sections are an integral part
of the mold, they are also known as green-sand
cores.
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Unfortunately, green-sand cores have a relatively low
level of strength.
If the protrusions are narrow or long, it might be
difficult to withdraw the pattern without breaking
them, or they may not have enough strength to even
support their own weight.
14.3 Cores and Core Making
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Dry-sand cores can be used to overcome
some of the cited difficulties.
These cores are made independent from the
remainder of the mold and are then inserted
into core prints to hold them in position.
The remaining sketches in Figure 14-21 show
dry-sand cores in the vertical and horizontal
positions.
14.3 Cores and Core Making
To function properly, cores must have the following
characteristics;
1. Sufficient hardness and strength (after baking or
hardening) to withstand handling and the forces of
the molten metal. Compressive strength should be
between 100 and 500 psi.
2. Sufficient strength before hardening to permit
handling in that condition.
3. Adequate permeability to permit the escape of gases.
Since cores are largely surrounded by molten metal,
they should possess exceptionally good permeability.
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14.3 Cores and Core Making
4. Collapsibility, After pouring, the cores must be weak
enough to permit shrinkage of the solidified casting
as it cools, thereby preventing cracking. In addition,
they must be easily removable from the interior of
the finished product via shakeout.
5. Adequate refractoriness, Since the cores are largely
surrounded by hot metal, they can become quite a
bit hotter than the adjacent mold material.
6. A smooth surface.
7. Minimum generation of gases when heated during
the pour.
14.3 Cores and Core Making
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Various techniques have been developed to enhance
the natural properties of cores and core materials.
Internal wires or rods can be used to 'impart
additional strength. Collapsibility can be enhanced by
making the cores hollow or by placing a material
such as straw in the center.
Enhanced collapsibility is particularly important in
steel castings, where a large amount of shrinkage is
observed.
All but the smallest of cores must be vented to
permit the escape of trapped and evolved gases.
Vent holes can be made by pushing small wires into
the core.
14.4 Other Expandable- Mold
Processes with Multiple-Use Patterns
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Plaster Mold Casting:
In plaster molding the mold material is plaster of
paris (also known as calcium sulfate or gypsum), with
various additions to improve strength, permeability,
and castability.
The mold material is first mixed with water, and the
resultant slurry is immediately poured over a pattern
and allowed to set.
Hydration of the plaster produces a hard solid that
can then be stripped from the pattern, which is
usually made from metal.
Plaster Mold Casting
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Table 14.4 summarizes the features of Plaster Mold Casting.
Ceramic Mold Casting
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Ceramic mold casting is similar to plaster
mold casting, except that the mold can now
withstand the higher-melting-point metals.
Cope-and-drag molds are formed around
withdrawable patterns (which can be metal or
wood), using a ceramic slury as the mold
material.
As with the plaster process, ceramic molding
can produce thin sections, fine detail, and
smooth surfaces and eliminate a considerable
amount of finish machining.
Ceramic Mold Casting
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Table 14.4 summarizes the features of Ceramic Mold
Casting.
Expendable Graphite Molds
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For metals such as titanium which tend to
react with many of the more common mold
materials, powdered graphite can be
combined with cement, starch, and water and
compacted around a pattern.
The pattern is then removed and the mold is
fired at I800°F (1OOO°C) to consolidate the
graphite.
After pouring, the mold is broken to remove
the casting.
Rubber-Mold Casting
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Several types of artificial elastomers are available
that can be compounded in liquid form and then
poured over a pattern to form a semi-rigid mold.
The molds are sufficiently flexible to permit stripping.
Unfortunately, rubber molds are suitable only for
small castings of low-melting-point materials.
Wax patterns for investment casting can be made as
well as finished castings of plastics
and metals which can be poured at temperatures
below 5OO°F(260°C).
14.5 EXPENDABLE-MOLD PROCESSES USING
SINGLE-USE PATTERNS
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Investment Casting:
While investment casting is actually a very old
process and has been performed by dentists and
jewelers for a number of years.
Developments and demands in the aerospace
industry, such as rocket components and jet engine
turbine blades, required high-precision complex
shapes from high-melting-point metals that are not
readily machineable.
Investment casting offers almost unlimited freedom
in both the complexity of shapes and types of
materials.
Investment Casting
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Table 14.6 summarizes features of investment
casting.