The molten metal is not directly poured into the mould cavity because it may cause mould erosion.China precision machining Molten metal is poured into a pouring basin which acts as a reservoir from which it moves smoothly in to sprue. The pouring basin is also able to stop the slag from entering the mould cavity by means of a skimmer or skim core as shown in Fig.5.27. It hold back the slag sand dirt which floats on the top and only allow the clean metal underneath it into the sprue. The pouring basin may be cut into the cope portion directly or a separate dry sand pouring basin may be prepared and used as
shown in Fig.5.27. the molten metal in the pouring basin should be full during the pouring operation, otherwise a funnel is likely to form through which atmospheric air and slag may enter the mould cavity.
(a) Green sand
(b) Dry sand Fig. 5.27 Pouring basin
One of the walls of the pouring basin is made inclined at about 45°to the horizontal. The molten metal is poured on this face such that metal momentum is absorbed and vortex formation is avoided. In some special cases the pouring basin may consist of partitions to allow for the trapping of the slag and maintaining constant metal height in the basin. Sprue
China precision machining Sprue is the channel through which the molten metal is brought into the parting plane where it enters the runners and gate to ultimately reach the mould cavity.
The molten metal when moving from the top of the cope to the parting plane grains in velocity and as a consequence requires a smaller area of cross section for the same amount of metal to flow at the top. If the sprue were to be straight cylindrical as shown in Fig. 5.28(a), then the metal flow not be full at the bottom, but some low pressure area would be created around the metal in the sprue. Since the sand mould is permeable, atmospheric air would be breathed into this low pressure area which would then be carried to the mould cavity. To eliminate this problem of air aspiration the sprue is tapered to gradually reduce the cross section as it moves away from the top of the cope as China precision machining hown in Fig.5.28(b).