Question

Shrinkage defects occur due to

• A. gas absorption
• B. volume contraction during solidification
• C. high pouring temperature
• D. low mould strength

Hint:

To distinguish casting defects in exams:
- Volumetric Contraction \(\rightarrow\) Shrinkage Cavities / Voids.
- Dissolved Hydrogen \(\rightarrow\) Pinholes / Gas Porosity.
- Poor Gating / Low Temp \(\rightarrow\) Cold Shuts / Misruns.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
This question asks for the fundamental physical cause of shrinkage defects (such as internal voids, porosity, or open pipe defects) in metal castings.

Step 2: Key Formula or Approach:
As molten metal cools and transforms from liquid to solid, it experiences a change in density. The volumetric shrinkage \( \Delta V \) during solidification is given by: \[ \Delta V = V_{\text{liquid}} - V_{\text{solid}} \] Because the crystal structure of the solid is more tightly packed than the liquid phase, \( V_{\text{solid}} \lt V_{\text{liquid}} \) for almost all metals, resulting in a volume contraction of \( 2-7\% \).

Step 3: Detailed Explanation:

• Three Stages of Contraction:
1.

Liquid Contraction: Contraction of the liquid as its temperature drops from pouring to liquidus.
2.

Solidification Shrinkage: Contraction during the phase change from liquid to solid. This is the stage that directly causes shrinkage cavities and macro-porosity because the solidifying regions shrink away from the remaining liquid.
3.

Solid Contraction: Contraction of the solid metal as it cools to room temperature (this affects the final dimensions and requires patternmaker's shrinkage allowance).

• Incorrect Options:
- Gas absorption leads to gas porosity (pinholes or blowholes), not shrinkage cavities.
- High pouring temperature can increase the amount of liquid shrinkage but does not directly cause the phase-change contraction itself.
- Low mold strength causes mold wall movement (swelling).


Step 4: Final Answer:
Shrinkage defects in casting are caused by the volumetric contraction that occurs during the phase transformation from liquid to solid.
Therefore, the correct choice is option (B).