Introduction to Casting
Types of Casting
Liquefying the Material
Viscosity and Fluidity
Casting Metals and Plastics
Casting Microstructure and Defects
Casting the Food Mixer Gearwheel
Once you have the liquid the properties that determine whether or not you can pour it depends on the viscosity or fluidity of the liquid material. The physical property that determines the ‘runniness’ of liquid is called viscosity. This varies with temperature and is not all that useful for describing how well a mould will be filled if the temperature of the liquid is falling as it runs into the cold mould. Water and most liquids at room temperature have low viscosities, so can be poured easily, as can thermoset precursors. Molten thermoplastics, freshly-mixed concrete and clays have much higher viscosities. In the casting of metals a more useful property is fluidity, which takes into account not only the viscosity changes but also the effects of cooling rate, surface tension of oxide films and the temperature range over which the alloy filling the mould actually freezes. Eutectic alloys have a high fluidity as they melt at a single temperature. Many of the alloys used for casting products are based on eutectic alloys.