Question

Which of the following gating ratios corresponds to a pressurized gating system during casting?

• A. 1:2:2
• B. 1:3:3
• C. 1:0.75:0.5
• D. 1:4:4

Hint:

To instantly recognize gating systems:
- Area decreases along the flow path (\( A_s \gt A_r \gt A_g \)) \(\rightarrow\) Pressurized System (e.g., \( 1:0.75:0.5 \)).
- Area increases along the flow path (\( A_s \lt A_r \lt A_g \)) \(\rightarrow\) Unpressurized System (e.g., \( 1:2:2 \) or \( 1:3:3 \)).

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
This question asks to identify the gating ratio that represents a pressurized gating system in metal casting.
A gating ratio defines the relationship between the cross-sectional areas of different parts of the gating system to control the flow rate of molten metal.

Step 2: Key Formula or Approach:
The gating ratio is expressed as:
\[ A_s : A_r : A_g \]
where:
\( A_s \) is the cross-sectional area of the sprue exit.
\( A_r \) is the total cross-sectional area of the runner(s).
\( A_g \) is the total cross-sectional area of the gate(s) entering the mold cavity.

Step 3: Detailed Explanation:

• Pressurized Gating System:
- In a pressurized system, the cross-sectional area decreases progressively from the sprue to the gates (i.e., \( A_s \gt A_r \gt A_g \)).
- This restriction creates backpressure, which keeps the gating channels completely filled with liquid metal.
- This limits air aspiration (the drawing in of gases) and reduces oxide formation.
- The ratio \( 1 : 0.75 : 0.5 \) clearly represents a decreasing area sequence, indicating a pressurized gating system.
- A primary disadvantage is that the metal enters the mold cavity at high velocity, which can cause turbulent flow and mold erosion.

• Unpressurized Gating System:
- In an unpressurized system, the area increases from the sprue to the gates (e.g., \( 1:2:2 \) or \( 1:3:3 \)).
- This keeps entering velocities low, preventing turbulence, but increases the risk of air aspiration if the system is not kept completely full.


Step 4: Final Answer:
A pressurized gating system requires a decreasing area ratio.
The ratio \( 1:0.75:0.5 \) satisfies this condition.
Therefore, the correct choice is option (C).