CORE AND CORE MAKING

Cores are sand structures used to create hollow sections, internal channels, or complex cavities in a casting. They are placed inside the mould before molten metal is poured. Since molten metal cannot fill the area occupied by the core, a hollow shape is formed in the final casting. After solidification, the core is removed or broken away, ensuring the desired internal geometry without additional machining.

MATERIALS USED IN CORE MAKING

The strength, collapsibility, and durability of a core depend on the binders used to hold the sand grains together. The main types of binders include:

● Water-soluble binders (2-4% by weight)

● Oil binders (1-3% by weight)

● Pitch and resin binders (1-35% by weight)

BINDERS

Binders are classified into two main types:

1. Inorganic Binders – These include clay-based binders like Kaolinite, Ball Clay, Fire Clay, Limonite, Fuller’s Earth, and Bentonite. However, clay alone cannot create strong bonds without moisture in the sand.
2. Organic Binders – These include dextrin, molasses, cereal binders, linseed oil, and synthetic resins like phenol-formaldehyde and urea-formaldehyde. Organic binders are preferred because they offer both strength and collapsibility. A strong core can withstand molten metal pressure, while a collapsible core breaks away easily after casting, preventing defects.

CORE PREPARATION PROCESS

1. Mixing Binder and Sand – The binder is poured and mixed into the moulding sand to prepare core sand.
2. Filling the Core Box – The prepared core sand is transferred into a core box (a mould designed to shape the core).
3. Compaction (Ramming) – The core sand is rammed into the core box to ensure uniform density and strength. Proper ramming prevents air pockets, weak spots, and collapse during casting.
4. Creating Vent Holes – A vent hole is made in the core to allow trapped gases to escape during metal pouring. Without venting, gas porosity and blow holes may form inside the casting.
5. Hardening the Core – In CO₂-based core hardening, carbon dioxide (CO₂) is pumped into the core, reacting with sodium silicate to form a solid, glass-like bond. This process eliminates the need for baking, making it faster and more efficient.
6. Core Removal – Once hardened, the core box is rammed or vibrated to loosen and remove the core without damage.