Foundry feeding systems — what is the primary function of a riser in a casting mould with respect to volumetric shrinkage during solidification?

Introduction / Context:
During solidification, most alloys undergo volumetric contraction. If liquid metal is not supplied to regions that are freezing, shrinkage cavities or dispersed porosity will form. Risers (feeders) are integral features in mould design to maintain soundness by compensating for this shrinkage.

Given Data / Assumptions:

• A conventional sand mould with a gating system and one or more risers.
• Alloy exhibits positive solidification shrinkage.
• Directional solidification from thin sections toward the riser is intended.

Concept / Approach:

A riser is a reservoir of molten metal thermally and geometrically designed to remain liquid longer than the casting sections it feeds. As the casting solidifies and contracts, the riser supplies additional metal through feeding paths to avoid void formation. The riser must freeze last (modulus and insulation considerations) and be properly located to ensure effective feeding, after which it is removed by a finishing operation.

Step-by-Step Solution:

Verification / Alternative check:

Soundness evaluation (radiography, sectioning) shows reduced shrinkage porosity when risers are correctly designed and placed.

Why Other Options Are Wrong:

(a) and (d) describe gating functions (sprue, runners, ingates), not feeding. (b) is partially true but incomplete; the essence is feeding to compensate shrinkage (option c).

Common Pitfalls:

Using oversized risers that raise yield loss; placing risers without thermal analysis, leading to ineffective feeding paths.

Final Answer:

feed the molten metal to the casting in order to compensate for the shrinkage