Definition of Clay:

Clay is a fine-grained natural material with particle sizes smaller than 2 microns. It primarily consists of clay minerals, mainly aluminium silicates, and acts as a binding agent in moulding sand.

Role of Clay in Moulding Sand:

Moulding sand typically contains 2% to 50% clay. The presence of clay provides the following characteristics: ● Binding Strength: Clay helps in holding the sand particles together, making the mould strong enough to retain its shape. ● Plasticity: When mixed with water, clay becomes plastic, meaning it can be moulded into various shapes without cracking. ● Moisture Retention: Due to its flake-like structure, clay has a large surface area, allowing it to hold and absorb moisture effectively. ● Effect on Permeability: If the clay content is too high, it reduces permeability, which means gases cannot escape easily, leading to defects in castings.

Types of Clay Components in Moulding Sand:

1. True Clay: This component provides the actual binding strength in the moulding sand.
2. Fine Silt: These are small particles that do not contribute to binding strength but are present in natural sands.

Why is Clay Content Measurement Important?

Base sand obtained from a natural source contains some amount of clay, but this amount may not always be suitable for mould-making. Too much or too little clay can affect the quality of the mould. ● If clay content is too high, the mould becomes less permeable, trapping gases and causing casting defects. ● If clay content is too low, the mould loses strength, leading to breakage during handling.

To achieve the optimal clay percentage, the clay content test is performed.

Principle of Clay Content Test:

The test works on the principle that when clay particles are suspended in water, they react with sodium hydroxide (NaOH) to form a precipitate. The clay content is determined by separating, drying, and weighing the precipitate. This helps in deciding whether to add or remove clay from the base sand for proper mould-making.