CASTING DEFECTS
The following are the major defects, which are likely to occur in sand castings:
• Gas defects
• Shrinkage cavities
• Molding material defects
• Pouring metal defects
• Mold shift
Gas 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.
Shrinkage Cavities
• 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
• Cuts and washes,
• Scab
• 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.
Scab
• This defect occurs when a portion of the face of a mould lifts or breaks down and the
recess thus made is filled by metal.
• When the metal is poured into the cavity, gas may be disengaged with such violence as
to break up the sand, which is then washed away and the resulting cavity filled with
metal.
• The reasons can be: ‐ too fine sand, low permeability of sand, high moisture content of
sand and uneven mould ramming
Metal penetration
• 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 penetration.
Fusion
• 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.
Swell
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.
Inclusions
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 defect.
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 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.
• 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.
Mold Shift
The mold shift defect occurs when cope and drag or molding boxes have not been properly
aligned.
Metallurgical defects
• Hot tears or hot cracking, cause of this defect is that stresses and strains built up during
solidification are too high compared to the actual strength of the semisolid material.
This type of defects occurs in the lower part of the solidification range, close to the
solidus, when the alloy has a wide solidification temperature range and a small amount
of liquid, when the solid fraction is more than 0.9, the hot tearing is easy to occur.
Proper mould design prevents this type of defect.
• Hot spots are areas on the surface of casting that become very hard because they
cooled more quickly than the surrounding material.
