Jawaharlal Nehru Technological University Hyderabad B.Tech II PRODUCTION TECHNOLOGY Previous Question Paper (March - 2021 - R18)

Composite moulds are casting moulds constructed by combining two or more different moulding materials to form the mould cavity. Typically, a high-quality, expensive facing sand is used to line the pattern surface, while a cheaper backing sand or material fills the remainder of the mould.

They are used to achieve superior surface finish and dimensional accuracy on the casting while simultaneously reducing overall production costs. By using a specialized facing material, foundries can improve the refractory properties and surface quality of the casting without the expense of using high-grade sand throughout the entire mould volume.

These moulds are commonly used in the production of large-sized castings, intricate components requiring high surface integrity, and in scenarios where specific thermal properties are needed at the metal-mould interface to prevent defects like sand fusion or metal penetration.

The differences between natural sand and synthetic sand are primarily based on their composition, consistency, and control over moulding properties:

• Composition: Natural sand is found in nature and contains a specific percentage of clay (typically 5-20%) as a binder. Synthetic sand is prepared by mixing clean, washed silica sand with specific amounts of additives, such as bentonite clay and water, to achieve desired properties.
• Consistency: Synthetic sand offers high uniformity and consistency in grain size and distribution, whereas natural sand often contains impurities and variable clay content, leading to inconsistent mould quality.
• Control: In synthetic sand, properties like permeability, green strength, and moisture content can be precisely controlled by adjusting the ratio of additives. Natural sand has limited adjustability, as the binder is inherent to the sand.
• Permeability: Synthetic sand generally provides higher permeability because the grains are cleaner and more uniform, allowing gases to escape more easily during pouring. Natural sand often has lower permeability due to the presence of fine impurities and excess clay.
• Cost and Maintenance: Synthetic sand is more expensive to prepare and requires mechanical mulling equipment for consistent mixing, whereas natural sand is cheaper but requires more frequent replacement or addition of binders to maintain performance.