ABSTRACT
Castings which are made where velocities everywhere in the mould never exceed the critical velocity are consistently strong, with high fatigue resistance, and are leak-tight. The speed requirement automatically excludes conventional pressure die-castings, since the filling speeds are over an order of magnitude in excess of the critical velocity. In gravity-filled running systems the requirement to reduce bubbles in the liquid stream during the filling of the casting calls for offset stepped basins, or other advanced basin designs. Pumped systems such as the Cosworth Process, or low-pressure filling systems into sand moulds or dies, are highly favoured as having the potential to avoid the entrainment of bubbles if the processes are carried out under proper control. Damage to the micro- and macro-structure of the casting can occur if the solidification time of the casting is commensurate with the time taken for convection to become established and start extensive remelting.
