JNTU B.Tech III FABRICATION PROCESSES Previous Question Paper (September - 2021 - R16)

Casting allowances are extra dimensions provided on a pattern to compensate for physical changes occurring during the solidification and cooling of the metal. The primary allowances are:

1. Shrinkage Allowance: Metals contract upon cooling from liquid to solid state. Patterns are made oversized to ensure the final casting meets the required dimensions.
2. Machining (Finish) Allowance: Extra material is added to surfaces that require machining to achieve a smooth finish or precise dimensions, as cast surfaces are often rough.
3. Draft Allowance: A slight taper is provided on vertical surfaces of the pattern to allow for its easy removal from the sand mold without damaging the mold walls.
4. Distortion (Camber) Allowance: For castings with complex shapes prone to warping during cooling, the pattern is intentionally distorted in the opposite direction to ensure the final shape is correct.
5. Shake (Rapping) Allowance: Since the pattern is rapped or vibrated to loosen it from the mold, the mold cavity becomes slightly enlarged. The pattern is made slightly smaller to compensate for this.

Moulding sand must possess specific properties to produce sound castings. The key properties are:

• Refractoriness: The ability of the sand to withstand high molten metal temperatures without fusing or softening. High silica content improves this property.
• Permeability: The capacity of the sand to allow gases and steam generated during pouring to escape through the mold. This prevents blowholes and porosity in the casting.
• Cohesiveness (Bond Strength): The ability of sand particles to stick together. This is essential for the mold to retain its shape and withstand the pressure of the molten metal.
• Flowability: The ability of the sand to flow into intricate corners and patterns under pressure, ensuring a dense and uniform mold.
• Collapsibility: The ability of the mold to break down easily after the metal solidifies, allowing the casting to contract freely and preventing hot tears or cracks.
• Adhesiveness: The ability of the sand to stick to the sides of the moulding box, ensuring the mold remains intact during handling.