Directional solidification in castings To encourage metal to freeze progressively from the thin sections toward the riser, which combination of foundry aids is commonly employed?

Introduction / Context:
Directional solidification reduces shrinkage defects by guiding the freezing front toward a properly located feeder (riser). This requires accelerating solidification in some regions and retarding it in others.

Given Data / Assumptions:

• Conventional sand or permanent mould casting with a riser.
• Thin and thick sections present in the casting.
• Objective: sound casting without internal shrinkage cavities.

Concept / Approach:
Chills are high-conductivity inserts placed against thick sections to extract heat faster, causing earlier freezing at desired spots. Padding (insulating or exothermic sleeves around risers or local hot spots) slows cooling, keeping the riser molten longest so that it can feed interdendritic shrinkage.

Step-by-Step Solution:
Promote early freezing where needed → apply chills to thick sections.Maintain molten metal in the riser → use insulating/exothermic sleeves (padding) to delay solidification at the feeder.Result: solidification front moves from extremities toward the riser (directional solidification).

Verification / Alternative check:
Thermal simulations or chill-trial castings verify freezing sequence: chills show earlier skin formation; exothermic riser sleeves keep feeder molten longer, ensuring proper feeding.

Why Other Options Are Wrong:
Chills and chaplets (option a): chaplets support cores; they do not control solidification. Chaplets and padding (option c) omit the chilling needed for thick sections. Using all aids in every case (option d) is unnecessary and can introduce defects. Larger gates alone (option e) do not guarantee directional solidification.

Common Pitfalls:
Placing chills indiscriminately causing cold shuts; neglecting venting; undersized risers that freeze too soon despite padding.

Final Answer:
Chills and padding (insulating/exothermic sleeves)