porosity and shrinkage

Dear Abid

The existance of cavities or porous system during cast processing is due to the poor methodology of casting and the properties of the melt (temperature, gas inclusions, salts contamination etc). To avoid pore generation, you use casting operation from downward upwards through a lateral feeding channel, overheat the molten metal about 150°C over its melting point, the boring flow rate should be slow, the die should be adequatly dry and other precautions should be managed.

Prof,Dr. Mahmoud Rabah

Just look at the defects.
The shrinkage defects are due to the stresses developed during the cooling down of the liquid metal, and the resultant volume contraction requirements not being met. These are usually almost always in form of cracks.
The porosity (and also blow holes) are due to the dissolved gases in liquid metal which are released during solidification of metal. It may also happen due to improper mould drying etc.
These two are never confusable if you undesrtand the causes which is as mentioned above. Even the cavities formed by shrinkage will not be nodular in shape.