The following are the major defects, which are likely to occur
in sand castings
- Shrinkage cavities
- Pouring metal defects
- Mold shift
|Fig Casting Defects|
A condition existing in a casting caused by the trapping of gas
in the molten metal or by mold gases evolved during the pouring
of the casting. The defects in this category can be classified
into blowholes and pinhole porosity. Blowholes are spherical or
elongated cavities present in the casting on the surface or
inside the casting. Pinhole porosity occurs due to the
dissolution of hydrogen gas, which gets entrapped during
heating of molten metal.
The lower gas-passing tendency of the mold, which may be due to
lower venting, lower permeability of the mold or improper
design of the casting. The lower permeability is caused by
finer grain size of the sand, high percentage of clay in mold
mixture, and excessive moisture present in the mold.
- Metal contains gas
- Mold is too hot
- Poor mold burnout
These are caused by liquid shrinkage occurring during the
solidification of the casting. To compensate for this, proper
feeding of liquid metal is required. For this reason risers are
placed at the appropriate places in the mold. Sprues may be too
thin, too long or not attached in the proper location, causing
shrinkage cavities. It is recommended to use thick sprues to
avoid shrinkage cavities.
Molding Material Defects
The defects in this category are cuts and washes, metal
penetration, fusion, and swell.
Cut and washes
These appear as rough spots and areas of excess metal, and are
caused by erosion of molding sand by the flowing metal. This is
caused by the molding sand not having enough strength and the
molten metal flowing at high velocity. The former can be taken
care of by the proper choice of molding sand and the latter can
be overcome by the proper design of the gating system.
When molten metal enters into the gaps between sand grains, the
result is a rough casting surface. This occurs because the sand
is coarse or no mold wash was applied on the surface of the
mold. The coarser the sand grains more the metal
This is caused by the fusion of the sand grains with the molten
metal, giving a brittle, glassy appearance on the casting
surface. The main reason for this is that the clay or the sand
particles are of lower refractoriness or that the pouring
temperature is too high.
Under the influence of metallostatic forces, the mold wall may
move back causing a swell in the dimension of the casting. A
proper ramming of the mold will correct this defect.
Particles of slag, refractory materials, sand or deoxidation
products are trapped in the casting during pouring
solidification. The provision of choke in the gating system and
the pouring basin at the top of the mold can prevent this
Pouring Metal Defects
The likely defects in this category are
- Mis-runs and
- Cold shuts.
A mis-run is caused when the metal is unable to fill the mold
cavity completely and thus leaves unfilled cavities. A mis-run
results when the metal is too cold to flow to the extremities
of the mold cavity before freezing. Long, thin
sections are subject to this defect and should be avoided in
A cold shut is caused when two streams while meeting in
the mold cavity, do not fuse together properly thus forming a
discontinuity in the casting. When the molten metal is poured
into the mold cavity through more-than-one gate, multiple
liquid fronts will have to flow together and become one
solid. If the flowing metal fronts are too cool,
they may not flow together, but will leave a seam in the
part. Such a seam is called a cold shut, and can be
prevented by assuring sufficient superheat in the poured metal
and thick enough walls in the casting
The mis-run and cold shut defects are caused either by a
lower fluidity of the mold or when the section thickness of the
casting is very small. Fluidity can be improved by changing the
composition of the metal and by increasing the pouring
temperature of the metal.
The mold shift defect occurs when cope and drag or
molding boxes have not been properly aligned.
The defects in a casting may be due to pattern and
moulding box equipment, moulding sand, cores,gating system or
molten metal. Some of the defects are:
1: Mould shift
It results in a mismatching of the top and the bottom
parts of the casting , usually at the parting line.
It is an enlargement of the mould cavity by molten metal
pressure resulting in localized or general enlargement of the
3: Fins and Flash
These are thin projections of the metal not intended as a
part of casting. These usually occurs at the parting line of
4: Sand Wash
It usually occurs near the in the gates as rough lumps on
the surface of a casting.
It is a crack or breakage in the casting on the surface
of the work piece, which results from un equal contraction of
the metal during solidification.
6: Hot Tear
It is an internal or external ragged discontinuously in
the metal casting resulting just after the metal has
7: Sand Blow or Blow Hole
It is smooth depression on the outer surface of the
casting work piece.
8: Honeycombing or Slag holes
These are smooth depression on the upper surface of the
casting. They usually occur near the ingates.
These are patches of sand on the upper surface of the
10: Cold Shut and Misruns
These happens when the mould cavity is not completely
filled by the molten and insufficient material or metal.
11: Run-outs and Bust-outs
These permit drainage of the metal from the cavity and
result in incomplete casting.