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Casting | Types of Casting Defects.

Casting-

A mould cavity is created out of sand or some permanent material and liquid metal is poured into this cavity and the product is taken out after solidification.

Types of Casting Defects

1) Gas Defects
 2) Moulding material Defects
3) Metallurgical Defects 
4) Pouring metal Defects

1. Gas Defects:-

These defects occur due to improper gas passing tendency of the mould. These defects are further classified into few types
(i) Air inclusions: - Air trapped in the final casting product. It is caused due to improper ventilation. These defects can be avoided by choosing appropriate pouring temperature and improving gating practice by reducing turbulence.
(ii) Open blows: - Spherical or elongated cavities formed on the surface of the casting product
(iii) Blow holes: - Cavities formed inside the final casting product
These open/blow holes are generally caused by the moisture present in the mould cavity or core it is also caused due to lower permeability and lower ventilation of mould. It can be avoided by providing proper ventilation.

2. Moulding Material Defects: - 

These defects are caused by the characteristics of the moulding sand material these are classified into few types: -
(i) Run out- Leaking of molten metal out of the mould. It is generally caused by faulty making of mould or faulty moulding flask.
(ii) Fusion – Brittle and glassy appearance on the casting product due to fusion of sand grains with molten metal.
These defects are caused by lower refractoriness of moulding sand or too high pouring temperature of molten metal. These can be avoided by selecting appropriate type of sand and amount of bentonite
(iii) Cuts and washers- The rough spot or area formed by excess metal present in the casting. This type of defect is caused by weak strength of moulding sand or high speed of flowing molten metal. It can be avoided by selecting proper moulding sand and using appropriate moulding method by increasing size of gates or using multiple ingates.

3. Pouring metal Defects:- 

These are the defects caused by the properties of molten metal being poured. These are of few types 
(i) Slag inclusion- It is the presence of slag in casting product along with molten metal. It is generally caused by the addition of flux in the molten metal this defect can be avoided by adoption of some of the slag trapping methods like pouring basin screens and runner extensions.
(ii) Misruns- This defect is caused due to low fluidity of molten metal and less thickness of the casting section, misruns are the cavities left unfilled by the molten metal. This defect can be avoided by increasing fluidity of the molten metal and by selecting proper casting design.

4. Metallurgical Defects:- 

These defects are caused by the properties of the casting product. These are further classified into few types: -
(i) Hot tear- This is the defect caused by the unwanted cooling stresses developed in the casting product which may rupture the product it is mainly caused by the poor casting design.
(ii) Hot spots- This is the defect caused by very hard white cast iron formed on the surface of casting made up of grey cast iron and small amount of silicon. This defect can be avoided by chilling of casting.


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