Question

Cold shut occurs when

• A. two metal streams fail to fuse
• B. gas entrapment occurs
• C. solidification shrinkage occurs
• D. sand collapses

Hint:

To distinguish between:
- Misrun: The metal solidifies before completely filling the mold cavity (incomplete casting).
- Cold Shut: The mold cavity is completely filled, but two metal streams failed to fuse, leaving a weak seam.
Both are remedied by increasing the pouring temperature and pouring speed.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
This question asks for the physical scenario that leads to the casting defect known as a "cold shut."
A cold shut is a major casting defect that severely compromises the mechanical strength of the component.

Step 2: Key Formula or Approach:
The formation of a cold shut is primarily a thermal and fluid-dynamics issue.
It occurs when the fluidity of the pouring metal is insufficient relative to the length and thickness of the mold channels.

Step 3: Detailed Explanation:

• Failure to Fuse:
- During casting, molten metal is poured into the gating system and split into multiple flow paths (streams) to fill different parts of the mold cavity.
- These streams must eventually meet and weld together to form a continuous solid piece.
- If the pouring temperature is too low, or if the pouring speed is too slow, the front of each stream cools rapidly below its liquidus temperature.
- Oxide films form on the advancing liquid surfaces.
- When the two cold, sluggish streams meet, they do not have enough thermal energy to dissolve these surface oxides and merge.
- This results in a discontinuous boundary line or seam (a cold shut) that remains in the final casting.

• Other Options:
- Gas entrapment causes blowholes or pinhole porosity, not cold shuts.
- Solidification shrinkage causes shrinkage cavities or piping.
- Sand collapse causes wash, scab, or mold drop defects.


Step 4: Final Answer:
A cold shut is structurally defined as the defect formed when two streams of liquid metal meet but fail to fuse completely due to premature cooling.
Therefore, option (A) is the correct answer.