Core making in foundry — which moulding sand is preferred for cores? In sand casting, separate core assemblies are used to form internal cavities. Which type of sand mix is commonly used specifically for making cores?

Introduction / Context:
Cores are inserts placed inside the mould cavity to create internal passages or undercuts in a casting. Unlike the mould, which may be made in green sand, cores must possess adequate strength at handling and at pouring temperature, good collapsibility, and surface finish suited to the internal features.

Given Data / Assumptions:

• Cores are baked (or chemically hardened) before use.
• Binder system should provide hot strength and easy shake-out.
• Question seeks the traditional sand type referenced in many syllabi.

Concept / Approach:
“Oil sand” (core sand) is silica sand bonded with drying oils (linseed, etc.) and additives; it is rammed into core boxes and baked to develop strength. While modern shops also use CO₂-sodium silicate or resin-bonded sands, the conventional textbook answer associates cores with oil-bonded core sand distinct from green sand used for moulds.

Step-by-Step Solution:

Verification / Alternative check:
Foundry manuals list oil-bonded core sands and shell cores as common core-making systems, contrasted with green sand moulds.

Why Other Options Are Wrong:

• Green sand: primarily for mould cavity; insufficient hot strength for most cores.
• Dry sand/loam sand: used for large moulds or special shapes, not the standard core mix.
• CO₂ sand only: modern method, but the question aims at the classic core material.

Common Pitfalls:
Assuming the same sand mix serves both mould and core; neglecting core venting and collapsibility.

Final Answer: