Sand casting
Sand casting, the most widely used casting process, utilizes
expendable sand molds to form complex metal parts that can be
made of nearly any alloy. Because the sand mold must be
destroyed in order to remove the part, called the casting, sand
casting typically has a low production rate. The sand casting
process involves the use of a furnace, metal, pattern, and sand
mold. The metal is melted in the furnace and then ladled and
poured into the cavity of the sand mold, which is formed by the
pattern. The sand mold separates along a parting line and the
solidified casting can be removed. The steps in this process are
described in greater detail in the next section.
Sand casting overview
Sand casting is used to produce a wide variety of metal
components with complex geometries. These parts can vary greatly
in size and weight, ranging from a couple ounces to several
tons. Some smaller sand cast parts include components as gears,
pulleys, crankshafts, connecting rods, and propellers. Larger
applications include housings for large equipment and heavy
machine bases. Sand casting is also common in producing
automobile components, such as engine blocks, engine manifolds,
cylinder heads, and transmission cases.
The process cycle for sand casting consists of six main
stages, which are explained below.
1. Mold-making - The first step in the sand casting
process is to create the mold for the casting. In an expendable
mold process, this step must be performed for each casting. A
sand mold is formed by packing sand into each half of the mold.
The sand is packed around the pattern, which is a replica of the
external shape of the casting. When the pattern is removed, the
cavity that will form the casting remains. Any internal features
of the casting that cannot be formed by the pattern are formed
by separate cores which are made of sand prior to the formation
of the mold. Further details on mold-making will be described in
the next section. The mold-making time includes positioning the
pattern, packing the sand, and removing the pattern. The
mold-making time is affected by the size of the part, the number
of cores, and the type of sand mold. If the mold type requires
heating or baking time, the mold-making time is substantially
increased. Also, lubrication is often applied to the surfaces of
the mold cavity in order to facilitate removal of the casting.
The use of a lubricant also improves the flow the metal and can
improve the surface finish of the casting. The lubricant that is
used is chosen based upon the sand and molten metal temperature.
2. Clamping - Once the mold has been made, it must be
prepared for the molten metal to be poured. The surface of the
mold cavity is first lubricated to facilitate the removal of the
casting. Then, the cores are positioned and the mold halves are
closed and securely clamped together. It is essential that the
mold halves remain securely closed to prevent the loss of any
material.
3. Pouring - The molten metal is maintained at a set
temperature in a furnace. After the mold has been clamped, the
molten metal can be ladled from its holding container in the
furnace and poured into the mold. The pouring can be performed
manually or by an automated machine. Enough molten metal must be
poured to fill the entire cavity and all channels in the mold.
The filling time is very short in order to prevent early
solidification of any one part of the metal.
4. Cooling - The molten metal that is poured into the
mold will begin to cool and solidify once it enters the cavity.
When the entire cavity is filled and the molten metal
solidifies, the final shape of the casting is formed. The mold
can not be opened until the cooling time has elapsed. The
desired cooling time can be estimated based upon the wall
thickness of the casting and the temperature of the metal. Most
of the possible defects that can occur are a result of the
solidification process. If some of the molten metal cools too
quickly, the part may exhibit shrinkage, cracks, or incomplete
sections. Preventative measures can be taken in designing both
the part and the mold and will be explored in later sections.
5. Removal - After the predetermined solidification time
has passed, the sand mold can simply be broken, and the casting
removed. This step, sometimes called shakeout, is typically
performed by a vibrating machine that shakes the sand and
casting out of the flask. Once removed, the casting will likely
have some sand and oxide layers adhered to the surface. Shot
blasting is sometimes used to remove any remaining sand,
especially from internal surfaces, and reduce the surface
roughness.
6. Trimming - During cooling, the material from the
channels in the mold solidifies attached to the part. This
excess material must be trimmed from the casting either manually
via cutting or sawing, or using a trimming press. The time
required to trim the excess material can be estimated from the
size of the casting's envelope. A larger casting will require a
longer trimming time. The scrap material that results from this
trimming is either discarded or reused in the sand casting
process. However, the scrap material may need to be
reconditioned to the proper chemical composition before it can
be combined with non-recycled metal and reused.
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