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Chapter: Mechanical : Manufacturing Technology : Metal Casting Process

Casting Defects

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

Casting Defects


Casting defects

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

 

a)Misruns

 

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.

 

e) Microporosity

 

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.

 

 


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