Question

Shell moulding uses

• A. resin coated sand
• B. clay sand
• C. cement mould
• D. graphite mould

Hint:

Key features of Shell Molding:
- High dimensional accuracy.
- Excellent surface finish.
- Suitable for casting thin-walled parts.
- Uses thermosetting phenolic resin-coated sand.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
This question asks for the primary molding material used to construct the mold in the shell molding casting process.
Shell molding is a precision casting technique developed to produce high dimensional accuracy and excellent surface finishes.

Step 2: Key Formula or Approach:
Unlike conventional green sand casting, which relies on clay and water as binders, shell molding uses a dry mixture of fine silica sand and a thermosetting polymer binder.

Step 3: Detailed Explanation:

• Material Composition:
- Fine, dry silica sand is coated with a thermosetting phenolic resin (typically \( 3-6\text{ wt}\% \) phenol-formaldehyde) and a catalyst (such as hexamethylenetetramine).
- This is known as resin-coated sand.

• Process Sequence:
- A metal pattern is heated to \( 150^{\circ}\text{C}-350^{\circ}\text{C} \).
- The resin-coated sand is dumped onto the heated pattern.
- The heat melts the resin, which flows between the sand grains and cures, forming a hard, bonded shell of sand (typically \( 5-10\text{ mm} \) thick) around the pattern.
- The excess sand is dumped off, and the cured shell is stripped from the pattern and clamped together with a matching half shell to form the complete mold.

• Comparison:
- Clay sand (green sand) is used in conventional green sand molding.
- Cement and graphite molds are used for specialty applications and permanent molds, respectively.


Step 4: Final Answer:
Shell molding is characterized by its use of dry, resin-coated sand to create thin, strong molds.
Thus, the correct option is (A).