Gating of the casting,Hot tears in casting, Shrinkage in casting,Segregation in casting,Gases in casting

a)      Gating of the casting:

The term gating system refers to all passageways through which the molten metal passes to enter the mold cavity.

The gating system is composed of:

1)      Pouring cup and basin

2)      Sprue

3)      Runner

4)      Gates

5)      Riser

b)     Hot tears in casting:

1)      Hot tears are cracks which appear in a castings during solidification from the melt or shortly after solidification while the casting is still hot.

2)      Hot tears are internal or external cracks having ragged edges.

3)      Immediately after solidification of the casting develops sufficiently high tensile or shear stresses, the metal fails with a resulting hot tears.

c)      Shrinkage in casting:

1)      Almost all cast metals shrinks or contract volumetrically during or after solidification.

2)      Different metals shrinks at different rates because shrinkage is the property of the cast metal or alloy.

3)      The metal shrinkage depends upon:

a)      The cast metal or alloy

b)      Pouring temperature of the metal or alloy

c)      Casting dimensions (size)

d)     Casting design aspects

e)      Molding conditions (i.e. mold materials and molding methods employed)

d)     Segregation in casting:

The result of difference in composition within an individual grain or from one part of the casting to another is known as coring or segregation.

Types of segregation:

1)      Micro-segregation

2)      Macro-segregation

e)      Gases in casting:

Gases are absorbed in molten metals from various sources such as:

1)      Furnace charges, i.e. wet metal charge, fluxes etc.

2)      Furnace refractories if they are not properly preheated.

3)      Furnace atmosphere which consists of water vapor, CO, CO2, SO2 and normal atmospheric gases such as oxygen and nitrogen.