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

In the casting process, the covering passage (commonly referred to as the gating system) is an intricate system used to control the flow of liquid metal into the mold cavity. One critical function of the gating system is to avoid the aspiration of air. Let's explore why this is important and examine the options given in the question:

1. Decrease the waste of cast metal: The gating system is designed to direct the flow of metal into the mold efficiently, but its primary function is not specifically to decrease metal wastage. Proper design can minimize waste, yet this is not the primary role of a covering passage.
2. Avoid the aspiration of air (Correct Answer): The covering passage helps to prevent the air from being drawn into the mold during the pouring process. Aspiration of air can lead to defects like air pockets and porosity in the final casting, affecting its strength and integrity. Hence, this is a vital function of the gating system.
3. Increase rate of feeding: While the design of the passage may influence the rate of feeding, it is not primarily aimed at increasing it. Instead, it balances the rate with turbulence control to reduce defects.
4. Quickly break off the protecting portion of the casting: This is not a function of the covering passage in the casting system. The break-off of parts usually involves additional post-processing techniques.

In summary, the main role of the covering passage in the context of the provided options is to avoid the aspiration of air. This function is crucial as it directly impacts the quality of the final casting by minimizing defects like porosity and ensuring a high-integrity product.