Jawaharlal Nehru Technological University Hyderabad B.Tech III FABRICATION PROCESSES Previous Question Paper (March - 2021 - R16)

Moulding sand must possess specific properties to ensure high-quality castings. Key properties include: Refractoriness, the ability to withstand high molten metal temperatures without fusing; Permeability, which allows gases and steam to escape through the mould; Flowability, the capacity to move into intricate patterns under pressure; Cohesiveness (green strength), the ability to hold shape when moist; and Adhesiveness, the ability to stick to the moulding box walls.

Absence of these properties leads to defects: low permeability causes blowholes or pinholes due to trapped gas; poor refractoriness results in fusion or burn-on defects; and insufficient strength leads to mould erosion or crushing. Other common casting defects include misruns (incomplete filling), cold shuts (improper fusion of two metal streams), shrinkage cavities (due to inadequate feeding), hot tears (cracks during cooling), and sand inclusions.

The principle of a gating system is to deliver molten metal into the mould cavity at a controlled rate, minimizing turbulence, preventing air aspiration, and ensuring directional solidification. It must be designed to fill the cavity before the metal freezes while maintaining a pressure head to compensate for shrinkage.

The system consists of the following components in order: Pouring Basin (receives metal), Sprue (vertical channel), Sprue Well (reduces turbulence), Runner (distributes metal), and Ingate (entry point to the cavity). A typical design follows a tapered sprue to prevent air aspiration. A sketch would show the pouring basin at the top, leading into a tapered vertical sprue, connecting to a runner bar, and finally through the ingate into the mould cavity, with a riser placed at the highest point to act as a reservoir for molten metal during solidification.