Question

Why the sprue is tapered downward to reduce the area?

Answer 1

Any changes in direction or cross sectional area should make use of rounded corners. To avoid the aspiration the tapered sprues are designed in the gating systems. A sprue tapered to a smaller size at its bottom will create a choke which will help keep the sprue full of molten metal.

Answer 2

There are two reasons why the sprue pin is tapered:

The sprue hole should be tapered to prevent air from entering the castingThe sprue pin needs to be removed easily from the mold after the mold sets so that liquid metal can be introduced into the cast