Pouring is a process by which molten metal is transferred to the cast for cooling and solidification and thus be converted into final product. Pouring temperature is the temperature to which the molten metal has to be raised to before being poured into casts for cooling and setting. This pouring temperature must also take into account the heat loss and caused due to the transfer of metal through ladles, as a distance between furnace and cast has to be covered and also due to the heat absorbed by ladles.
The casting process can usually be done in permanent metal casts. However, due to repeated exposure to high temperature of molten metal, these casts have a limited life, or can be used for metals with low pouring temperature requirements. Therefore one of the main requirements of the casting process is refractoriness or in other words, the capability of cast to bear high temperatures of the molten metal without undergoing any changes in its physical properties.
This is a very important requirement in alloys with high melting point such as steel. However, this issue may be taken secondary in alloys with lower melting points. Where alloys with high melting point are being used, the moulds need to be lined with an insulating material with refractory properties so that the mould retains its shape and original characteristics.
Sand and ceramic material has a very high ability to withstand high temperatures of molten metals without undergoing a change in their properties. Therefore, they are used as coating material for moulds in which alloys with high temperatures have to be poured for cooling. Sand and ceramic can withstand temperatures as high as 1650-1820c. Also, sand retains the shape given to it when it is put into a mould. It also permits various gases to escape through its structure. Sand casting can be used in processing of low-temperature metals, such as iron, copper, aluminum, magnesium, and nickel alloys and also for high temperature metals where other mould material cannot be used.
Metal moulds have a limited capacity to withstand high temperatures. Metal moulds are used in processes like Die Casting (where molten metal is forced into steel moulds under high pressure) and permanent molding. These molds may change their physical properties in case molten metal above 1180C pouring temperature are processed in such moulds. For any alloy with temperature above this temperature, metal moulds are not suitable.