 These are used when the casting alloy cannot be used in hot @-@ chamber machines ; these include aluminium , zinc alloys with a large composition of aluminium , magnesium and copper . The process for these machines start with melting the metal in a separate furnace . Then a precise amount of molten metal is transported to the cold @-@ chamber machine where it is fed into an unheated shot chamber ( or injection cylinder ) . This shot is then driven into the die by a hydraulic or mechanical piston . The biggest disadvantage of this system is the slower cycle time due to the need to transfer the molten metal from the furnace to the cold @-@ chamber machine . 
