Sand casting has many defects that can occur due to the mould failing. The mould usually fails because of one of two reasons: the wrong material is used or it is improperly rammed.
The first type is mould erosion, which is the wearing away of the mould as the liquid metal fills the mould. This type of defect usually only occurs in sand castings because most other casting processes have more robust moulds. The castings produced have rough spots and excess material. The moulding sand becomes incorporated into the casting metal and decreases the ductility, fatigue strength, and fracture toughness of the casting. This can be caused by a sand with too little strength or a pouring velocity that is too fast. The pouring velocity can be reduced by redesigning the gating system to use larger runners or multiple gates. A related source of defects are drops, in which part of the moulding sand from the cope drops into the casting while it is still a liquid. This also occurs when the mould is not properly rammed.
The second type of defect is metal penetration, which occurs when the liquid metal penetrates into the moulding sand. This causes a rough surface finish. This is caused by sand particles which are too coarse, lack of mould wash, or pouring temperatures that are too high. An alternative form of metal penetration into the mould known as veining is caused by cracking of the sand.
If the pouring temperature is too high or a sand of low melting point is used then the sand can fuse to the casting. When this happens the surface of the casting produced has a brittle, glassy appearance.
A run out occurs when the liquid metal leaks out of the mould because of a faulty mould or flask.
s are a thin layer of metal that sits proud of the casting. They are easy to remove and always reveal a  underneath, which is an indentation in the casting surface. s are similar to buckles, except they are thin line indentations and not associated with scabs. Another similar defect is s, 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.
 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.