Porosity is usually discovered when a casting is being machined. As layers of metal are removed, voids and channels will become obvious. Porosity can be classified into three main categories Gas Porosity caused by gas formation and trapped air during the casting process.
Flow Porosity caused by unequal pressure on the molten metal during the casting process which causes flow porosity Shrink Porosity caused by changes in volume as the metal solidifies. One technique of addressing porosity is by impregnating the pores with an inert material which can withstand high temperature and vibration.