Foundry sands — permeability versus grain size State whether the following statement is correct: “If the moulding sand is too fine, its permeability will be high.”

Introduction / Context:Permeability of moulding sand determines how easily gases from moisture and binders can escape during pouring. The relationship between grain fineness and permeability is central to preventing gas-related defects like blows and pinholes in castings.

Given Data / Assumptions:

• Sand mixes contain sand grains, binder (e.g., clay), moisture, and additives.
• Permeability is the measure of air flow through a compacted sand specimen.
• Other variables (moisture, ramming density, binder) being equal.

Concept / Approach:Fine sand grains pack more closely, reducing pore size and, hence, decreasing permeability. Coarser sands leave larger voids and offer higher permeability. Foundry practice balances surface finish (improved by finer sands) against gas escape (improved by coarser sands) based on casting geometry and metal type.

Step-by-Step Solution:Identify variable: finer grains lead to smaller intergranular pores.Relate pores to air flow: smaller pores reduce permeability.Therefore, the claim that “too fine” implies “high permeability” is incorrect.Hence, select False.

Verification / Alternative check:Permeability number tests show inverse trends with grain fineness number (GFN): higher GFN (finer sand) generally yields lower permeability for similar compaction and moisture.

Why Other Options Are Wrong:Qualifiers about dry sand or zero binder do not reverse the fundamental geometric effect of grain size; venting helps, but the base statement still remains false.

Common Pitfalls:Overemphasizing surface finish benefits of fine sand and neglecting gas evacuation needs; always match sand grading to casting section thickness and binder system.

Final Answer:False