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Sand casting is the most widely used metal casting process in manufacturing. Almost all
casting metals can be sand cast. Sand castings can range in size from very small to extremely large. Some
examples of items manufactured in modern industry by sand casting processes are engine blocks, machine tool bases,
cylinder heads, pump housings, and valves, just to name a few.
Sand: Sand: Product of the disintegration of rocks over long periods of time.
Most sand casting operations use silica sand (SiO2). A great advantage of sand in manufacturing
applications is that sand is inexpensive. Another advantage of sand to manufacture products by metal casting processes,
is that sand is very resistant to elevated temperatures. In fact, sand casting is one of the few processes that can
be used for metals with high melting temperatures such as steels, nickel, and titanium. Usually sand used to manufacture a
mold for the casting process is held together by a mixture of water and clay. A typical mixture by volume could be
89% sand, 4% water, 7% clay. Control of all aspects of the properties of sand is crucial when manufacturing parts by
sand casting, therefore a sand laboratory is usually attached to the foundry.
Use Of Binder In Sand Casting:
A mold must have the physical integrity to keep its shape throughout the casting operation. For this reason,
in sand casting, the sand must contain some type of binder that acts to hold the sand particles together. Clay
serves an essential purpose in the sand casting manufacturing process, as a binding agent to adhere the molding
sand together. In manufacturing industry other agents may be used to bond the molding sand together in place
of clay. Organic resins, (such as phenolic resins), and inorganic bonding agents, (such as phosphate and sodium
silicate), may also be used to hold the sand together. In addition to sand and bonding agents, the sand mixture
to create the metal casting mold will sometimes have other constituents added to it in order to improve mold properties.
Types Of Sand Used In Sand Casting:
There are two general types of sand used in the manufacturing process of sand casting.
Naturally Bonded- Naturally bonded sand is less expensive but it includes organic impurities that reduce
the fusion temperature of the sand mixture for the casting, lower the binding strength, and require a higher
Synthetic Sand- Synthetic sand is mixed in a manufacturing lab starting with a pure (SiO2)
sand base. In this case, the composition can be controlled more accurately, which imparts the casting sand mixture
with higher green strength, more permeability, and greater refractory strength. For these reasons, synthetic sand
is mostly preferred in sand casting manufacture.
Properties Of A Sand Casting Mixture:
Type And Content Of Binder And Other Additives:
As mentioned, controlling the type and content of the sand binder and other additives is the key to controlling the
properties of the casting's mold sand mixture.
Moisture content affects the other properties of the mixture such as strength and permeability. Too much moisture
can cause steam bubbles to be entrapped in the metal casting.
Grain Size: This property represents the size of the individual particles of sand.
Shape of Grains: This property evaluates the shape of the individual grains of sand
based on how round they are. Less round grains are said to be more irregular.
Strength: The explanation of strength is, the ability of the sand casting mixture to hold its geometric shape under
the conditions of mechanical stress imposed during the sand casting process.
Permeability: The ability of the sand mold to permit the escape of air, gases,
and steam during the sand casting process.
Collapsibility: The ability of the sand mixture to collapse under force.
Collapsibility is a very important property in this type of casting manufacture. Collapsibility of the mold will
allow the metal casting to shrink freely during the solidification phase of the process. If the molding sand cannot
collapse adequately for the casting's shrinkage, hot tearing
or cracking will develop in the casting.
Flowability: The ability of the sand mixture to flow over and fill the sand casting
pattern during the impression making phase of the manufacturing process, more flowability is useful for a more
Refractory Strength: During the pouring of the molten metal in sand casting
manufacture, the sand mixture in the mold must not melt, burn, crack, or sinter. The refractory strength is
the ability of the mold sand mixture to withstand levels of extreme temperature.
Reusability: The ability of the mold sand mixture to be reused
to produce other sand castings in subsequent manufacturing operations.
When planning the manufacture of a particular casting, remember some properties of a sand casting mold mixture are
contradictory to each other. Tradeoffs in different properties are often needed to achieve a compromise that provides
a sand casting mold mixture with adequate properties for the specific part and casting application. There are some
things to consider when selecting a sand mixture for a manufacturing process. Small grain size enhances mold
strength, but large grain size is more permeable. Sand casting molds made from grains of irregular shape tend to
be stronger because of grain interlocking, but rounder grains provide a better surface finish. A sand casting
mold mixture with more collapsibility has less strength, and a sand casting mixture with more strength has less
Sand Conditioning For A Metal Casting Operation:
If the sand is being reused from a previous sand casting manufacturing process, lumps should be crushed and then
all particles and metal granules removed, (a magnetic field may be used to assist in this). All sand and
constituents should be screened. In industrial practice shakers, rotary screens, or vibrating screens, are
used in this process. Then continuous screw-mixers or mulling machines are used to mix the sand uniformly.
Types Of Molds Used In Sand Casting:
Green Sand Molds:
A green sand mold is very typical in sand casting manufacture, it is simple and easy to make,
a mixture of sand, clay and water. The term
green refers to the fact that the mold will contain moisture during the pouring of the casting.
Manufacturing Considerations And Properties Of Green Sand Molds:
- Has sufficient strength for most sand casting applications
- Good collapsibility
- Good permeability
- Good reusability
- Least expensive of the molds used in sand casting manufacturing processes
- Moisture in sand can cause defects in some castings, dependent upon the type of metal used in the
sand casting and the geometry of the part to be cast.
Dry Sand Molds:
Dry sand molds are baked in an oven, (at 300F - 650F for 8-48 hours), prior to the sand casting operation, in order to
dry the mold. This drying strengthens the mold, and hardens its internal surfaces. Dry sand molds are
manufactured using organic binders rather than clay.
Manufacturing Considerations And Properties Of Dry Sand Molds:
- Better dimensional accuracy of sand cast part than green sand molds
- Better surface finish of sand cast part than green sand molds
- More expensive manufacturing process than green sand production
- Manufacturing production rate of castings are reduced due to drying time
- Distortion of the mold is greater, (during mold manufacture)
- The metal casting is more susceptible to hot tearing because of the lower collapsibility of the mold
- Dry sand casting is generally limited to the manufacture of medium and large castings
Skin Dried Molds:
When sand casting a part by the skin dried mold process a green sand mold is employed, and its mold cavity surface
is dried to a depth of .5 - 1 inch. Drying is a part of the manufacturing process and is accomplished by use of
torches, heating lamps or some other means, such as drying it in air.
Manufacturing Considerations And Properties Of Skin Dried Molds:
- The cast part dimensional and surface finish advantages of dry sand molds are partially achieved
- No large oven is needed
- Special bonding materials must be added to the sand mixture to strengthen the mold cavity surface
Cold Setting Processes:
In industrial sand casting manufacture, sometimes non-traditional binders other than those used in the above
classifications of sand molds may be used. These binders may be made of a variety of things, such as
synthetic liquid resins. Conventional casting binders require heat to cure while these when mixed with the
sand, bond chemically at room temperature. Hence the term cold setting processes. Technically advanced,
these relatively recent
sand casting processes are growing in manufacturing. While more expensive than green sand molds, cold setting
processes provide good dimensional accuracy of the casting, and have high production applications.
Mold Setup For Sand Casting:
The setup of a sand mold in manufacturing involves using a pattern to create an impression of the part to
be sand cast within the mold, removal of the pattern, the placement of cores, (if needed), and the creation of
a gating system within the mold. The setup of a mold is covered in detail in
metal casting process.
A mold setup such as the one in Figure 6 could be typical in a
sand casting manufacturing operation.
A few different types of patterns may be used in the sand casting process.
This is a one piece pattern representing the geometry of the casting. It is an easy pattern to manufacture, but
determining the parting line between cope and drag is more difficult for the foundry worker.
The split pattern is comprised of two separate parts that when put together will represent the geometry of the
casting. When placed in the mold properly the plane at which the two parts are assembled should coincide with the
parting line of the mold. This makes it easier to manufacture a pattern with more complicated geometry. Also
mold setup is easier since the patterns placement relative to the parting line of the mold is predetermined.
Match Plate Pattern:
The match plate pattern is typically used in high production industry runs for sand casting manufacture. A match plate
pattern is a two piece pattern representing the casting, divided at the parting line, similar to the split
pattern. In the match plate pattern, however, each of the parts are mounted on a plate. The plates
come together to assemble the pattern for the sand casting process. The match plate pattern is more proficient and
makes alignment of the pattern in the mold quick and accurate.
Cope and Drag Pattern:
The cope and drag pattern is also typical in sand casting processes for high production industry runs. The cope and drag pattern is
the same as the match plate pattern in that it is a two piece pattern representing the casting and divided at the parting line.
Each of the two halves are mounted on a plate for easy alignment of the pattern and mold. The difference between the cope
and drag pattern and the match plate pattern is that in the match plate pattern the two halves are mounted together, where as
in the cope and drag pattern the two halves are separate. The cope and drag pattern enables the cope section of the mold, and
the drag section of the mold to be created separately and latter assembled before the pouring of the sand casting.
In industrial sand casting processes a gating system, (not shown), is often incorporated as part of the pattern,
particularly for a cope and drag pattern. Patterns can be
made of different materials, and the geometry of the pattern must be adjusted for shrinkage, machine finish,
and distortion. Pattern basics are covered in detail in
the patterns section.
Cores form the internal geometry of the casting. Cores are placed in the mold, and remain there during the pouring
phase of the sand casting process. The metal casting will solidify around the core. Core basics are covered in
detail in the cores section. Cores are made of the highest quality
sand and are subject to extreme conditions
during the sand casting operation. Cores must be strong and permeable; also, since the metal casting will shrink
onto the core, cores must have sufficient collapsibility. Sometimes a reinforcing material will be placed in
a sand casting core to enhance strength. The core may be manufactured with vents to facilitate the removal
The Sand Casting Operation:
The sand casting operation involves the pouring of the molten metal into the sand mold, the solidification of the
casting within the mold, and the removal of the casting. The casting operation is covered in detail on the
metal casting operation page.
Of specific interest to sand casting would be; the effect and dissipation of heat through the particular sand mold
mixture during the casting's solidification, the effect of the flow of liquid metal on the integrity of the mold,
(mold sand mixture properties and binder issues), and the escape
of gases through the mixture. Sand usually has the
ability to withstand extremely high temperature levels, and generally allows the escape of gases quite well.
Manufacturing with sand casting allows the creation of castings with complex geometry. Sand casting manufacture, however,
only imparts a fair amount of dimensional accuracy to the cast part.
After the sand casting is removed from the sand mold it is shaken out, all the sand is otherwise removed
from the casting, and the gating system is cut off the part. The part may then undergo further manufacturing
processes such as heat treatment, machining, and/or metal forming. Inspection is always carried out on the finished part
to evaluate the effectiveness and satisfaction of its manufacture.