Defects may occur due to one or more of the following reasons:
– Fault in design of casting pattern
– Fault in design on mold and core
– Fault in design of gating system and riser
– Improper choice of moulding sand
– Improper metal composition
– Inadequate melting temperature and rate of pouring
Some common defects in castings:
a) Misruns b) Cold Shut c) Cold Shot d) Shrinkage Cavity e) Microporosity f) Hot Tearing
It is a casting that has solidified before completely filling the mold cavity. Typical causes include
1) Fluidity of the molten metal is insufficient,
2) Pouring Temperature is too low,
3) Pouring is done too slowly and/or
4) Cross section of the mold cavity is too thin.
b) Cold Shut
A cold shut occurs when two portion of the metal flow together, but there is lack of
fusion between them due to premature freezing, Its causes are similar to those of a Misruns.
c) Cold Shots
When splattering occurs during pouring, solid globules of the metal are formed that
become entrapped in the casting. Poring procedures and gating system designs that avoid splattering can prevent these defects.
d) Shrinkage Cavity
This defects is a depression in the surface or an internal void in the casting caused by solidification shrinkage that restricts the amount of the molten metal available in the last region to freeze.
This refers to a network of a small voids distributed throughout the casting caused by
localized solidification shrinkage of the final molten metal in the dendritic structure. f) Hot Tearing
This defect, also called hot cracking, occurs when the casting is restrained or early stages
of cooling after solidification.