Difference between Blow hole and pin hole porosity
Blow hole : They appear as cavities in a casting. When they are visible on the upper surface of the castings, they are cooled open blows. These blows are normally rounded and have smooth walls.
Pin hole porosity : This defect occurs in the castings in the form of pin hole porosity or gas porosity. These porosities are caused by the gases absorbed by the molten metal. The gases commonly absorbed and hydrogen, oxygen and nitrogen, the later two forming oxides and nitrides respectively. It is mainly hydrogen which is responsible for the pin hole porosity.