Theory
Risers are only effective if three conditions are met: the riser cools after the casting, the riser has enough material compensate for the casting shrinkage, and the casting directionally solidifies towards the riser.
For the riser to cool after the casting the riser must cool more slowly than the casting. Chvorinov's rule briefly states that the slowest cooling time is achieved with the greatest volume and the least surface area; geometrically speaking, this is a sphere. So, ideally, a riser should be a sphere, but this isn't a very practical shape to insert into a mold, so a cylinder is used instead. The height to diameter ratio of the cylinder varies depending on the material, location of the riser, size of the flask, etc.
The shrinkage must be calculated for the casting to confirm that there is enough material in the riser to compensate for the shrinkage. If it appears there is not enough material then the size of the riser must be increased. This requirement is more important for plate-like shapes, while the first requirement is more important for chunky shapes.
Finally, the casting must be designed to produce directional solidification, which sweeps from the extremities of the mold cavity toward the riser(s). In this way, the riser can feed molten metal continuously to part of the casting that is solidifying. One part of achieving this end is by placing the riser near the thickest and largest part of the casting, as that part of the casting will cool and solidify last. If this type of solidification is not possible, multiple risers that feed various sections of the casting or chills may be necessary.
Read more about this topic: Riser (casting)
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