The efficiency, or yield, of a casting is defined as the weight of the casting divided by the weight of the total amount of metal poured. Risers can add a lot to the total weight being poured, so it is important to optimize their size and shape. Because risers exist only to ensure the integrity of the casting, they are removed after the part has cooled, and their metal is remelted to be used again. As a result, riser size, number, and placement should be carefully planned to reduce waste while filling all the shrinkage in the casting.
One way to calculate the minimum size of a riser is to use Chvorinov's rule by setting the solidification time for the riser to be longer than that of the casting. Any time can be chosen but 25% longer is usually a safe choice, which is written as follows:
or
Because all of the mold and material factors are the same for n. If a cylinder is chosen for the geometry of the riser and the height to diameter ratio is locked, then the equation can be solved for a diameter, which makes this method a simple way to calculate the minimum size for a riser. Note that if a top riser is used the surface area that is shared between the riser and the casting should be subtracted from the area on the casting and the riser.
Read more about this topic: Riser (casting)
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