The volume of the riser must always be greater than the volume of the casting in order to avoid premature solidification of the riser: True or False

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Explanation:

The elevators also well known as risers are only effective if three conditions are met: the elevator cools after casting, the elevator has enough material to compensate for the contraction of the melt, and the foundry solidifies directionally towards the elevator.

For the elevator or riser to cool after launch, the riser must cool more slowly than the launch. Chvorinov's rule states briefly that the slowest cooling time is achieved with the largest volume and the smallest surface area; geometrically speaking, this is a sphere. So, ideally, an elevator should be a sphere, but this is not a very practical way to insert into a mold, so a cylinder is used. The height / diameter of the cylinder varies depending on the material, the location of the riser, the size of the flask, etc.

The shrinkage should be calculated for the foundry to confirm that there is enough material in the lift to compensate for the shrinkage. If there seems to be insufficient material, the size of the elevator should be increased. This requirement is more important for plate-like forms, while the first requirement is more important for thick forms.

Finally, the foundry must be designed to produce a directional solidification, which extends from the extremities of the mold cavity towards the elevations. In this way, the elevator can feed molten metal continuously to part of the foundry that is solidifying. One part of achieving this goal is to place the elevator near the thickest and largest part of the mold, since that part of the mold will cool and solidify at the end. If this type of solidification is not possible, multiple elevators that feed several sections of the foundry or cooling may be necessary.