Pipes form at the surface of the casting and burrow into the casting, while caved surfaces are shallow cavities that form across the surface of the casting.Closed shrinkage defects, also known as shrinkage porosity, are defects that form within the casting. Isolated pools of liquid form inside solidified metal, which are called hot spots. The shrinkage defect usually forms at the top of the hot spots. They require a nucleation point, so impurities and dissolved gas can induce closed shrinkage defects. The defects are broken up into macroporosity and microporosity (or micro shrinkage), where macroporosity can be seen by the naked eye and microporosity cannot. Gas porosity
Gas porosity is the formation of bubbles within the casting after it has cooled. This occurs because most liquid materials can hold a large amount of dissolved gas, but the solid form of the same material cannot, so the gas forms bubbles within the material as it cools. Gas porosity may present itself on the surface of the casting as porosity or the pore may be trapped inside the metal, which reduces strength in that vicinity. Nitrogen, oxygen and hydrogen are the most encountered gases in cases of gas porosity. In aluminium castings, hydrogen is the only gas that dissolves in significant quantity, which can result in hydrogen gas porosity. For casting that are a few kilograms in weight the pores are usually 0.01 to 0.5 mm (0.00039 to 0.01969 in) in size.
Rattails are similar to buckles, except they are thin line indentations and not associated with scabs. Another similar defect is pulldowns, which are buckles that occur in the cope of sand castings. All of these defects are visual in nature and are no reason to scrap the workpiece. These defects are caused by overly high pouring temperatures or deficiencies of carbonaceous material.A swell occurs when the mould wall gives way across a whole face, and is caused by an improperly rammed mould.Burn-on occurs when metallic oxides interact with impurities in silica sands. The result is sand particles embedded in the surface of the finished casting. This defect can be avoided by reducing the temperature of the liquid metal, by using a mould wash, and by using various additives in the sand mixture. See also
Hydrogen gas porosity
Inclusions in aluminium alloys
Non-metallic inclusions for inclusions in steel
Porosity sealing

References
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