Question

In a casting, shrinkage occurs

• A. only during transformation from liquid to solid
• B. only after transformation from liquid solid
• C. before, during after transformation from liquid to solid
• D. only when the metal is in liquid state

Hint:

While shrinkage happens in three stages (liquid, solidification, solid), the term is often used in the context of casting defects to refer specifically to solidification shrinkage, as this is what causes voids. Always consider the context of the question.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Shrinkage in casting refers to the volumetric contraction that a metal undergoes as it cools from the molten state to room temperature. This is a critical phenomenon that must be managed to produce dimensionally accurate and sound castings. It occurs in three distinct stages.

Step 2: Detailed Explanation:
The three stages of shrinkage are:

• Liquid Shrinkage: The contraction of the liquid metal as it cools from the pouring temperature down to the temperature where solidification begins. This is compensated for by the gating system and riser design.

• Solidification Shrinkage: The contraction that occurs during the phase change from liquid to solid. This is the most critical type of shrinkage as it is the primary cause of porosity and voids. This is the shrinkage that risers are specifically designed to combat by feeding liquid metal into the solidifying casting.

• Solid Shrinkage: The contraction of the solidified casting as it cools from the solidification temperature down to room temperature. This is compensated for by making the pattern oversized, using a "shrinkage allowance."
The question and the provided answer choice seem to focus on the most problematic form of shrinkage that leads to internal defects. While shrinkage technically occurs in all three stages (making option C the most physically accurate description), the shrinkage that causes voids and is the main focus of feeding system design occurs *during* the transformation from liquid to solid. The provided answer key selects option (A), implying that the question is implicitly asking about the stage that causes internal voids.

Step 3: Final Answer:
Focusing on the type of shrinkage that leads to internal porosity and must be compensated for by risers, this occurs specifically during the transformation from liquid to solid.