Question

The function of a riser in casting is to

• A. increase cooling rate
• B. remove gases
• C. reduce mould temperature
• D. feed molten metal during solidification

Hint:

Riser Design Criteria:
1. Modulus Criterion: \( M_r \gt M_c \) (Riser solidifies last).
2. Volume Criterion: The riser must contain enough liquid volume to feed the entire contraction of the casting.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
This question asks for the primary engineering function of a "riser" (also known as a feeder) in a casting gating system design.

Step 2: Key Formula or Approach:
Because metals shrink during solidification, risers are added as external reservoirs of molten metal. To perform its function, the riser must solidify *after* the casting. By Chvorinov's Rule, the solidification time \( t_s \) is: \[ t_s = B \left(\frac{V}{A}\right)^2 \] where:
\( V \) is the volume, \( A \) is the surface area, and \( B \) is the mold constant. Therefore, the Modulus of the riser must be greater than the Modulus of the casting: \[ M_r \ge 1.2 \cdot M_c \]

Step 3: Detailed Explanation:

• Feeding Solidification Contraction:
- As the metal solidifies inside the mold cavity, it contracts (shrinkage).
- The riser acts as a reservoir of liquid metal, continuously feeding the casting to fill any voids formed by this contraction.
- Any shrinkage cavity is thus concentrated inside the riser rather than inside the critical part of the casting.
- Once solidified, the riser is cut off and recycled.

• Other Functions: While the riser does allow mold gases to escape and indicates when the mold is full, these are secondary benefits. Its primary designed purpose is feeding shrinkage.
- It does not increase the cooling rate (chills do) and does not reduce mold temperature.


Step 4: Final Answer:
The fundamental purpose of a riser is to provide liquid metal to compensate for volume contraction during solidification.
Thus, option (D) is the correct choice.