The mould, or die, that is used only one single time, is made from sand or natural/synthetic ground, sometimes conglomerated with a binder, or with sodium silicate solutions. The mix is then caused to react with a flow of CO2 to make it solid.
This technique is used for small quantity productions, or for very large castings, which cannot be produced using other processes.
When filling the sand mould with molten metal and during the solidification phase, defects can appear in the castings if the molten metal is not filtered properly, the feeders or sprues are not correctly sized, the heat cycle of the mould is not managed correctly, or the modification and degassing are not managed correctly.
That is why it is important to perform perfect filtering and cleaning of the molten alloy.