Molten metal flowing through the runner enters into mould cavity using gate.
As the poured molten metal is cooled in the mould cavity, the solidification happens. Since density of solid is greater than liquid, the size of solidified metal is lesser than size of the molten metal. As a result the size of the molten metal shrinks upon cooling and produces little bit small part than required size. In order to compensate this problem, riser stores some extra molten metal and supplies the same to mould cavity when there is a change in volume of molten metal (Shrinkage). Hence it is not possible to produce perfect casting without riser.