बड़ी तथा भारी ढलाई ______ बनायी जाती है।

Explanation:

Large and Heavy Castings:

• Large and heavy castings require specific moulding methods that can handle their size and weight effectively.

Green Sand Moulding:

• Commonly used for small to medium-sized castings.
• Uses a moist mixture of sand, clay, and water.
• Not ideal for very large and heavy castings due to strength and stability limitations.

Pit Moulding:

• Specifically designed for large castings.
• Mould is made directly in a pit or trench dug into the ground.
• Provides necessary support and stability for heavy and large castings.
• Can accommodate larger sizes compared to other methods due to the structural support of the pit.

Dry Sand Moulding:

• Mould is made from sand that has been dried to remove moisture.
• Provides better dimensional accuracy and surface finish than green sand moulding.
• Can be used for large castings but requires more preparation and is more expensive than pit moulding.

Pressure Moulding:

• Involves the application of pressure to form the mould.
• Generally used for small to medium-sized castings requiring high precision and surface finish.
• Not typically used for very large and heavy castings due to the complexity and equipment required.

Important Points

For manufacturing large and heavy castings, pit moulding is the most appropriate method due to its ability to provide the necessary support and accommodate the size and weight of the castings.