Overview
Sand
casting is an economical process for creating rough metal parts. Raw
castings are then machined to produce finished products or components.
Sand casting is the least expensive of all casting processes, including
die and investment casting. Sand Casting Post Process Description:
The Sand casting post process may require a longer lead time
for production at high output rates (1–20 pieces/hour/mold), but is
unsurpassed for large-part production. Sand has almost no upper limit
on part weight and minimum part weight ranges from 0.075–0.1 kg. The
sand is bonded together using clays or chemical binders. In most
operations, the sand can be recycled many times, requiring the addition
of only small amounts of sand each time.
Preparation of the sand mold is fast but requires a
pattern that can “stamp” out the casting template. Typically, sand
casting is used for processing low-temperature metals, such as iron,
copper, aluminum, magnesium, and nickel alloys. It can also be used for
high-temperature metals where other means would be impractical. The sand
casting post process is by far the oldest and best understood of all techniques. As
such, automation can easily be adapted to the production process,
although somewhat less easily to the design and preparation of forms.
The Sand Casting Process
In the sand casting process, a pattern is
made in the shape of the desired part. The pattern can easily be made
using PolyJet™ models.
A single piece or solid pattern is used for
simple designs. Patterns that are more complex are made in two parts,
and are called split patterns. This can also be designed in the CAD
level, and printed by Objet systems. The upper part of a split pattern
is called a cope, while the bottom section is called a drag. Where the
cope and drag separate is known as the parting line. Both solid and
split patterns can have cores inserted to complete the final part
shape. When making a pattern, it is necessary to taper the edges so the
pattern can be removed without breaking the mold.
Forming the Cavity during the sand casting post processIn the sand casting post process the pattern is housed in a box called the
flask, and then packed with sand. A binder helps harden the sand into a
semi-permanent shape. Once the sand mold is cured, the pattern is
removed. This leaves a hollow space in the sand in the shape of the
desired part. The pattern is made larger than the cast to allow for
shrinkage during cooling. Sand cores can then be inserted in the mold
to create holes and improve the casting’s overall shape. Simple
patterns are usually open on top, allowing molten metal to be poured
into them. Two-piece molds are clamped together. Molten metal is poured
into a pouring cup from where it travels down a sprue and into the
gating system. Vent holes are created to allow hot gases to escape
during the pour. Ideally, the pouring temperature of the molten metal
should be a few hundred degrees higher than the melting point, assuring
good fluidity. The temperature difference also prevents premature
cooling and the resulting voids and porosity. After the metal cools,
the sand mold is removed and the metal part is ready for additional
operations, such as cutoff and grinding.
Sprues and Runners in the sand casting post processThe molten material is poured into the
pouring cup, which is part of the gating system that supplies the
molten material to the mold cavity. The vertical part of the gating
system that is connected to the pouring cup is the sprue, and the
horizontal portions are called the runners. The multiple points where
the material is introduced to the mold cavity are called the gates.
Additionally there are extensions to the gating system, called vents
that provide the path for the built-up gases and the displaced air to
vent to the atmosphere.
The cavity is usually made oversized
to allow for metal contraction as it cools down to room temperature during the sand casting post process.
This is achieved by making the pattern oversized. To allow for
shrinking, the pattern must be oversized according to certain averaged
factors. There are linear factors that apply in each direction. These
shrinkage allowances are only approximate because the exact allowance
is determined by the shape and size of the casting. In addition,
different parts of the casting might require a different shrinkage
allowance.
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