Question

The riser is designed such that the melt in the riser solidifies

• A. After casting solidifies
• B. Before casting solidifies
• C. At the same time as casting solidifies
• D. Irrespective of the solidification of the casting

Hint:

The simple rule for risers is: "The riser must feed, not be fed." For this to happen, it must solidify last. This means it needs a larger solidification time, which is achieved with a higher V/A ratio compared to the casting.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
A riser, or a feeder, is a reservoir of molten metal attached to a casting mold. Its primary function is to compensate for solidification shrinkage. As the metal in the main casting cools and shrinks, the riser supplies additional molten metal to prevent shrinkage voids from forming.

Step 2: Detailed Explanation:
For a riser to perform its function effectively, it must remain molten longer than the casting it is feeding.

• If the riser solidifies

before or

at the same time as the casting, it can no longer supply liquid metal. The last parts of the casting to solidify will then be starved of metal, leading to the formation of shrinkage cavities.

• Therefore, the fundamental design principle for a riser is that its solidification time must be greater than the solidification time of the casting. The riser must be the last part of the entire system to freeze.

• This is achieved by applying Chvorinov's Rule, which states that solidification time is proportional to \( (V/A)^2 \), where V is the volume and A is the surface area. A riser is designed to have a larger Volume-to-Surface Area ratio than the casting, allowing it to cool more slowly and solidify last.

Step 3: Final Answer:
To successfully feed the casting and compensate for shrinkage, the riser must be designed to solidify after the main casting has completely solidified.