A Sand casting is used to make large
parts (typically Iron, but also Bronze, Brass, Aluminum). Molten metal
is poured into a mold cavity formed out of sand (natural or synthetic).
The processes of sand casting are discussed in this section, include
patterns, sprues and runners, design considerations, and casting
allowance. The cavity in the sand is formed by using a pattern (an
approximate duplicate of the real part), which is typically made out of
wood, sometimes metal. The cavity is contained in an aggregate housed in
a box called the flask. A core is a sand shape inserted into the mold
to produce the internal features of the part such as holes or internal
passages. Cores are placed in the cavity to form holes of the desired
shapes. Core print is the region added to the pattern, core, or mold
that is used to locate and support the core within the mold. A riser is
an extra void created in the mold to contain excessive molten material.
The purpose of this is to feed the molten metal to the mold cavity as
the molten metal solidifies and shrinks, and thereby prevents voids in
the main casting. In a two-part mold, which is typical of sand castings,
the upper half, including the top half of the pattern, flask, and core
is called cope and the lower half is called drag. The parting line or
the parting surface is a line or surface that separates the cope and
drag. The drag is first filled partially with sand, and the core print,
the cores, and the gating system place near the parting line. The cope
is then assembled to the drag, and the sand is poured on the cope half,
covering the pattern, core, and the gating system.
The sand is
compacted by vibration and mechanical means. Next, the cope is removed
from the drag, and the pattern is carefully removed. The object is to
remove the pattern without breaking the mold cavity. This is facilitated
by designing a draft, a slight angular offset from the vertical to the
vertical surfaces of the pattern. This is usually a minimum of 1° or 1.5
mm (0.060 in), whichever is greater. The rougher the surface of the
pattern, the more the draft to be provided. The 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 connected to the pouring cup is the sprue, and the
horizontal portion is called the runners, and finally to the multiple
points where it is introduced to the mold cavity 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 oversize to allow for the metal contraction as
it cools down to room temperature. This is achieved by making the
pattern oversize. To account for shrinking, the pattern must be made
oversize by these factors, on the average. These are linear factors and
apply in each direction. These shrinkage allowances are only approximate
because the exact allowance is determined by the shape and size of the China casting. In addition, different parts of the casting might require a different shrinkage allowance. Sand castings
generally have a rough surface, sometimes with surface impurities, and
surface variations. A machining (finish) allowance is made for this type
of defect.
OTTOMMO Sand Casting
plant has both manual and semi-automatic green/resin sand casting
production lines for castings with various weight and geometries.
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