Question

In casting process, the covering passage is used for feeding the liquid metal into the mould to

• A. Decrease the waste of cast metal
• B. Avoid the aspiration of air
• C. Increase rate of feeding
• D. Quickly break off the protecting portion of the casting

Hint:

Air aspiration is caused by low-pressure zones ($P \lt P_{\text{atm}}$) inside the gating channels. To prevent air from being sucked into the liquid metal stream through the sand mold, channels like the sprue are tapered to keep the fluid under positive pressure, ensuring a clean casting free of gas porosity.

Answer 1

The Correct Option is B

Concept: In metal casting, the gating system is a series of interconnected channels designed to guide molten metal smoothly from the pouring basin into the mold cavity. A major concern during pouring is Air Aspiration. Air aspiration occurs if the pressure at any point within the gating channels drops below atmospheric pressure ($P_{\text{local}} \lt P_{\text{atm}}$). This pressure drop creates a localized vacuum that draws surrounding gases and air through the porous sand mold into the molten metal stream. These trapped gases can react with the liquid metal or become trapped during solidification, causing casting defects such as blowholes, gas porosity, and internal oxides. To prevent air aspiration, the vertical channel known as the sprue must be designed with a specific geometric profile.

Step 1: Analyze the fluid dynamics of molten metal flow in a sprue.
Let's apply the continuity equation and Bernoulli's equation to the liquid metal falling under gravity down a vertical sprue channel:

• As the liquid metal falls, it accelerates due to gravity, increasing its velocity ($V$).

• According to the volumetric continuity equation ($A_1 V_1 = A_2 V_2$), as velocity increases, the cross-sectional area of the liquid stream naturally contracts or narrows.
If a straight, cylindrical vertical sprue is used, the accelerating metal stream will pull away from the channel walls, creating a low-pressure vacuum zone that causes air aspiration.

Step 2: Evaluate how channel geometry prevents aspiration.
To prevent this vacuum formation, the sprue channel must be designed with a tapered profile (wider at the top intake and narrower at the bottom exit) that matches the natural contraction of the falling liquid stream. This design ensures that the moving liquid metal remains in continuous contact with the channel walls, keeping the pressure throughout the passage above atmospheric pressure. This covered, tapered gating design prevents the aspiration of air and gases into the stream.

Step 3: Select the option that matches this design objective.
The primary functional purpose of configuring and covering these passages is to maintain positive pressure and avoid the aspiration of air, matching Option (B).