Defects are common phenomenon in any engineering process. There is some limitation in any process which causes defects. Proper designing and moulding can give a defect free cast, but due to some control limitation and human interference defects are generally occur. By the proper assistance on work we can minimise the casting defects. Generally, a proper casting product also has some defects. So, research is going in foundry industry to minimise these defects.
- These casting defects can be found out by the non-destructive testing methods which includes ultrasonic testing,
- radiography testing,
- magnetic particle testing,
- die penetration testing, etc.
The basic conditions which sparked the defects in casting or the favourable conditions of casting defects are –
- defect in design of pattern
- improper melting of metal
- defects in mould
- due to moulding material
- improper cooling of mould,
- defect in gating system and
- improper venting in mould.
These defects can be classified in three major categories.
BLOWHOLE
The first is blowhole – it is a gas hole or gas cavity which occurs at the surface or the internal part of casting. It is due to air get interrupted into the casting, due to poor ventilation. Mostly occur at surface which makes a circular of oval shaped gas hole. These defects can be seen after machining of surface.
T hese defect cause due to improper venting system excessive gas or moisture contained and due to low permeability of sand and high temperature of sand.
It is also due to low pouring temperature of metal
SCAR
Scar is generally occurring at flat surface and it is due to improper venting or permeability of sand in the blue is covered by the thin layer of metal.
SCAB / SLAG
This is also known as slag inclusion of sand inclusion and this is looked like as slag inside the cast and very difficult to remedy because generally it does not occur at single point. This defect generally occurs due to defective mould or core low binding strength of moulding material or sand. It is also due to low permeability of sand the
PENETRATION
fourth one is penetration and this defect occurs when fluidity of molten metal is high.
When mould is porous and does not enough stronger to resist molten metal this defect occur the liquid metal penetrates the mould and mixed with sand and this gives uneven casting surface.
COLD SHOT
Occurs when the molten metal flow through two different paths.
When the two stream of molten metal comes in contact and solidify before they mix completely, some bubbles form between them and this phenomenon is known as cold shut and it is also known as cold lap.
MISMATCH
This defect is due to mismatch of cope and drag.
It is also known as shift. When the cope shifts relative to drag known as mould shift and when drag shift known as cope shift.
It is due to the bouncy of the molten metal loose box pins etc.
Miss runs.
Miss runs is a phenomenon when the cast materials solidify before the proper filling of mould.
This result presence of incomplete cavity filling and it is due to thin wall thickness improper gating system slow pouring damaged pattern and poor fluidity of molten metal.
HOT TEAR
A hot tear or crack is generally not visible because the casting has not separated into fragments.
This is occurring due to uneven cooling and improper position of chills.
This is generally occurring where abrupt section change and this is occurred during solidification of casting when the solid portion have not sufficient strength to resist tensile force.
SHRINKAGE
This defect generally arises during volume contraction during solidification and it is occur when the liquid metal does not sufficient to compensate Volume shrinkage.
This is due to insufficient size of riser, improper direction solidification of casting, improper location of chills or in gates.
BUCKLE
The formation of uneven line on the cast surface is known as buckle and generate due to failure of sand surface or expansion of sand surface due to heat of molten metal.
PINHOLE
Pinholes are similarly to blow holes but smaller into size and these are generally present at surface or subsurface and in large numbers, this is due to improper ventilation for gas and low permeability of moulding material.
WARPAGE
It is the defect or deformation in a casting that occur during or after solidification due to different rates of solidification of different sections of casting. So, stresses are set up in adjoining walls resulting in warpage these area next snd last one as well as this defect generated due to insufficient strength of mould when metal flow to the mould due to liquid metal pressure, the wall of the mould cavity damage it is also due to rigidity of the pattern is insufficient then it cannot withstand with the ramming pressure and the wall of cavity damage.
These defects can be removed by proper casting techniques.
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