JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III FOUNDRY TECHNOLOGY Previous Question Paper (November/December - 2020 - R16)

Pattern allowances are essential modifications made to the dimensions of a pattern to compensate for the physical changes that occur during the metal casting process, ensuring the final casting meets the required specifications. These allowances are categorized as follows:

1. Shrinkage Allowance: Most metals contract during cooling from the molten state to room temperature. To prevent the final casting from being undersized, the pattern is made larger. This is compensated by using a shrink rule, which is scaled according to the specific metal's contraction rate (e.g., steel shrinks more than cast iron).
2. Machining (Finish) Allowance: Castings often require subsequent machining to achieve desired surface finish or dimensional accuracy. Extra material is added to the pattern surfaces that will be machined, ensuring the casting is not undersized after the removal of the surface layer.
3. Draft (Taper) Allowance: To allow for the easy removal of the pattern from the sand mold without damaging the mold walls, a slight taper is provided on all vertical surfaces of the pattern. This ensures that the pattern can be withdrawn cleanly by minimizing friction against the sand.
4. Distortion (Camber) Allowance: Some castings, particularly those with long, thin, or U-shaped sections, tend to warp or distort during cooling due to uneven cooling rates or internal stresses. To counteract this, the pattern is intentionally distorted in the opposite direction so that the final casting assumes the correct shape upon cooling.
5. Shake (Rapping) Allowance: Before removing a pattern from the mold, it is often rapped or shaken to loosen it from the sand. This action slightly enlarges the mold cavity. To compensate for this, the pattern is made slightly smaller than the required final dimensions so that the resulting enlarged cavity remains accurate.

Moulding sands are primarily classified based on their source, composition, and usage in the foundry. The main types include:

• Green Sand: A mixture of silica sand, clay (10-20%), and moisture (6-8%). It is used in the moist state and provides good permeability and plasticity.
• Dry Sand: Green sand that has been dried or baked in an oven after the mould is prepared, resulting in higher strength and resistance to metal erosion.
• Loam Sand: A mixture with high clay content (up to 50%) and water, used for large castings where the mould is shaped by sweeping.
• Facing Sand: A high-quality mixture of new silica sand and clay, applied directly to the pattern surface to ensure a smooth casting finish.
• Backing Sand: Used to fill the bulk of the moulding box; it is usually recycled sand and does not require high refractoriness or fine grain size.
• System Sand: Sand used in mechanized foundries that is continuously cleaned and reconditioned.
• Parting Sand: Fine, clay-free sand used to prevent the cope and drag from sticking together.

The important properties of moulding sand essential for producing sound castings are:

1. Permeability: The ability of the sand to allow gases and steam to escape through the mould during pouring. High permeability prevents blowholes.
2. Refractoriness: The capacity of the sand to withstand high temperatures without fusing or softening. It is determined by the silica content.
3. Cohesiveness (Bond Strength): The ability of sand particles to stick together. It is divided into green strength (when moist) and dry strength (after drying).
4. Flowability: The ability of the sand to flow into intricate patterns and corners under pressure, ensuring accurate reproduction of the pattern shape.
5. Adhesiveness: The ability of the sand to stick to the sides of the moulding box or flask.
6. Collapsibility: The ability of the sand to yield during the contraction of the cooling metal, preventing hot tears or cracks in the casting.