Question

Compared to top gating, the effect(s) of bottom gating in sand mold casting is/are

• A. reduced melt oxidation
• B. reduced mold erosion
• C. enhanced melt oxidation
  (D) enhanced mold erosion
• D. enhanced mold erosion

Hint:

Remember this simple principle: Turbulence in casting is bad. Top gating creates high turbulence (splashing). Bottom gating minimizes turbulence (gentle filling). Therefore, bottom gating reduces the problems caused by turbulence, namely oxidation and mold erosion.

Answer 1

The Correct Option is A, B

Step 1: Understanding the Concept:
This question compares two different designs for a gating system in sand casting: top gating and bottom gating. The gating system is the network of channels that delivers molten metal to the mold cavity. The design significantly affects the quality of the final casting.
- Top Gating: Molten metal is poured directly from the top into the mold cavity.
- Bottom Gating: Molten metal enters the mold cavity at the bottom and fills upwards.
Step 2: Detailed Analysis of Gating Systems:
Let's analyze the effects of bottom gating compared to top gating:
- Melt Oxidation: In top gating, the metal falls a significant distance, splashing and creating a great deal of turbulence as it enters the mold. This turbulence exposes a large surface area of the molten metal to the air or gases within the mold, leading to enhanced melt oxidation and gas pickup. In bottom gating, the metal flows gently into the bottom of the mold and rises smoothly, minimizing splashing and turbulence. This gentle filling process significantly reduces melt oxidation. Therefore, statement (A) is correct and (C) is incorrect.
- Mold Erosion: The high-velocity, turbulent flow in a top gating system causes the falling stream of metal to impinge directly on the mold sand at the bottom of the cavity. This can dislodge sand particles, a defect known as mold erosion. These dislodged sand particles can then become trapped in the casting as sand inclusions. In bottom gating, the metal enters with lower velocity and fills the mold from the bottom up. This avoids the direct impact of a falling stream on the mold surfaces, thereby reducing mold erosion. Therefore, statement (B) is correct and (D) is incorrect.
Step 3: Why This is Correct:
Bottom gating is specifically designed to promote quiescent (calm, non-turbulent) filling of the mold cavity. This gentler filling directly leads to less mixing with air (reduced oxidation) and less mechanical damage to the mold walls (reduced erosion). Thus, (A) and (B) are the primary advantages of a bottom gating system over a top gating system.