The riser is designed such that the melt in the riser solidifiesa)Befo...
Explanation:
In casting, a riser is a reservoir that is designed to supply molten metal to the casting cavity as it solidifies and shrinks. The purpose of the riser is to ensure that there is sufficient molten metal available to compensate for the shrinkage that occurs during solidification.
The riser is designed such that the melt in the riser solidifies after the casting solidifies. This is because the solidification of the casting creates a heat sink that draws heat away from the molten metal in the riser. If the melt in the riser solidified before the casting, there would not be enough molten metal available to compensate for the shrinkage. On the other hand, if the melt in the riser solidified at the same time as the casting, it would not be able to supply additional molten metal to compensate for the shrinkage.
By designing the riser to solidify after the casting, the molten metal in the riser can continue to supply additional material to the casting as it solidifies and shrinks. This ensures that there is sufficient molten metal available to fill any voids or shrinkage cavities that may form during solidification.
Additionally, by allowing the riser to solidify after the casting, any defects or impurities that may be present in the riser can be isolated and prevented from affecting the quality of the casting. This is because the solidified riser can be removed and discarded without affecting the integrity of the casting.
In summary, the riser is designed to solidify after the casting to ensure that there is sufficient molten metal available to compensate for shrinkage and to isolate any defects or impurities.