Question

Select the correct statement for the riser design

• A. The size of riser should be designed for maximum possible volume but should maintain a solidification time less than that of casting.
• B. The size of riser should be designed for minimum possible volume but should maintain a solidification time longer than that of casting.
• C. The size of riser should be designed for maximum possible volume but should maintain a solidification time longer than that of casting.
• D. The size of riser should be designed for minimum possible volume but should maintain a solidification time less than that of casting.

Hint:

The key to riser design is balancing the need for sufficient molten metal to compensate for shrinkage with the desire to minimize material waste. Chvorinov's rule and the concept of modulus are crucial tools in achieving this balance.

Answer 1

The Correct Option is B

Step 1: Understand the function of a riser in casting.
A riser is a reservoir built into a mold to prevent cavities due to shrinkage as the metal solidifies. The molten metal in the riser flows into the mold cavity to compensate for the volume contraction during solidification. Step 2: Consider the solidification time of the riser and the casting.
For a riser to effectively feed molten metal to the casting during solidification, it must remain molten longer than the casting. This ensures that liquid metal is available to compensate for shrinkage as the casting solidifies. The solidification time \( t_s \) of a casting or riser is governed by Chvorinov's rule: $$t_s = C \left( \frac{V}{A} \right)^n$$ where:
\( V \) is the volume of the casting or riser
\( A \) is the surface area of the casting or riser
\( C \) is the mold constant (dependent on the mold and metal material)
\( n \) is an exponent, typically around 2
To ensure the riser solidifies last, its solidification time \( (t_s)_{riser} \) must be greater than the solidification time of the casting \( (t_s)_{casting} \): $$(t_s)_{riser}>(t_s)_{casting}$$$$C \left( \frac{V}{A} \right)_{riser}^n>C \left( \frac{V}{A} \right)_{casting}^n$$$$\left( \frac{V}{A} \right)_{riser}>\left( \frac{V}{A} \right)_{casting}$$The ratio \( V/A \) is known as the modulus \( M \). So, the modulus of the riser should be greater than the modulus of the casting:$$M_{riser}>M_{casting}$$ Step 3: Consider the volume of the riser.
While the riser needs to solidify later than the casting, it also represents wasted metal that needs to be cut off after solidification. Therefore, it is desirable to design the riser with the minimum possible volume that still ensures it remains molten long enough to feed the casting. Step 4: Evaluate the given statements.
Option (1) suggests maximum volume and shorter solidification time, which is incorrect.
Option (2) suggests minimum volume and longer solidification time, which aligns with the requirements for an efficient and effective riser.
Option (3) suggests maximum volume and longer solidification time. While longer solidification is desired, minimizing volume is also important for material efficiency.
Option (4) suggests minimum volume and shorter solidification time, which would not allow the riser to feed the casting effectively. Therefore, the correct statement is that the size of the riser should be designed for minimum possible volume but should maintain a solidification time longer than that of casting.