Casting defects

CLASSIFICATION :

1. Surface defects

   1. Blow

   2. Scar

   3. Blister

   4. Drop

   5. Scab

   6. Penetration

   7. Buckle

2. Internal defects

   1. Blow holes

   2. Porosity

   3. Pin holes

   4. Inclusion dross

3. Visible defects

   1. Wash

   2. Rat tail

   3. Swell

   4. Mis run

   5. Cold shut

   6. Hotbtera

   7. Shrinkage

BLOW

• Blow is comparatively large cavity produced by gases that displace molten metal from convex surface.

SCAR

• It is sometimes found on the flat casting surface. It’s a shallow blow.

BLISTER

• This is a scar covered by the thin layers of the metal.

DROP

• Sometimes sand particles dropping out of the cope get embedded on the top surface of a casting. Once removed these leave small angular holes is termed as dirts.

PENETRATION

• It seems as an uneven and rough external surface of the casting. It may be caused when the sand has too high permeability, large grain size, and low strength.

BUCKLE

• It refers to a long fairly shallow broad depression at the surface of a casting of a high temperature metal.

• Due to very high temperature of the molten metal expansion of the thin layered of the sand at the mold face takes place. As this expansion is obstructed by the flux the mold tends to bulge out forming a V shape..

POROSITY

• Porosity occurs in materials especially castings as they modify state from liquid to solid throughout the manufacturing process.

• Casting porosity has the shape of surface and core imperfections that either effects the surface finish or as a leak path for gases and liquids.

• The poring temperature ought to be maintained properly to reduce porosity.

BLOWHOLE

• Blowholes are smooth round holes that are clearly perceptible on the surface of the casting. To prevent blowholes moisture content in sand should be adjusted, sand of proper grain size ought to be used.

DROSS

• The lighter impurities are appearing on the top of the cast surface is called the dross. It can be taken care of at the pouring stage by using items such as a strainer.

PIN HOLES

• Pin holes are tiny blow holes appearing just below the casting surface.

INCLUSIONS

• Inclusions are the non-metallic particles in the metal matrix.

SHRINKAGE

• Shrinkage of molten metal as it solidifies is an important issue in casting. It will reduce the 5-10% volume of the cast. Ought to design part and mold to take this amount into consideration. Shrinkage defect will be reduced by decreasing the quantity of walls and increasing the draft angle.

WASH

• It is a low projection on the drag surface of a casting commencing near the gate. It is caused by the erosion of sand due to high velocity liquid metal.

RAT TAIL

• It is a long shallow angular depression found in a thin casting. The cause is similar to buckle.

SWELL

• Swell is the deformation of vertical mould surface due to hydrostatic pressure caused by moisture in the sand.

COLD SHUT AND MIS-RUN

• A cold shut is a defect in which a discontinuity is formed due to the imperfect fusion of two streams of metal in the mold cavity.

• The reasons for cold shut or mis-run may be too thin sections and wall thickness, improper gating system, damaged patterns, slow and intermittent pouring , poor fluidity of metal caused by low pouring temperature, improper alloy composition.

HOT TEAR

• Hot tears are internal or external ragged discontinuities or crack on the casting surface caused by rapid contraction occurring immediately after the metal solidified.

• Hot tear may be caused when the mold and core have poor collapsibility or when the mold is too hard causing the casting to undergo severe strain during cooling.

• Incorrect pouring temperature and improper placement of gates and risers can also create hot tears.

SHIFT

• A shift leads to a mismatch of the sections of a casting sometimes as a parting line.

• Misalignment is common cause of shift. This defect will be prevented by ensuring proper alignment of the pattern for die parts, molding boxes and checking of pattern flux locating pins before use.