In casting, feed refers to the supply of liquid metal to a solidifying region within the mold cavity to compensate for the volumetric contraction that occurs as the metal cools and transitions from liquid to solid.
Why Feeding is Essential
Metals and alloys typically shrink significantly in volume when they solidify. If this shrinkage is not compensated for by additional liquid metal flowing into the solidifying areas, voids or pores (shrinkage defects) will form within the casting, compromising its structural integrity and quality.
The Process of Feeding
Feeding is the natural or facilitated process where liquid metal from hotter, still-liquid parts of the casting or from designated reservoirs (like risers) flows into cooler, solidifying areas. This flow fills the space left by the shrinking metal.
Understanding the Feed-Path
The direction and ease with which this liquid metal flows is critical for effective feeding. According to casting principles:
- The feed-path at any given location inside a casting is the direction along which liquid metal flows to an adjacent solidifying region, to compensate its volumetric contraction.
- The feed-path at any point is along the local maximum temperature gradient.
- This direction is in turn normal to the solidification front (the boundary between the liquid and solid phases).
This means that liquid metal naturally flows towards the coolest areas undergoing solidification, following the steepest temperature drop.
Importance for Casting Quality
Effective feeding is paramount for producing high-quality castings free from internal shrinkage defects. Proper mold and casting design, including the strategic placement of risers, gating systems, and sometimes chills, is crucial to ensure that feeding can occur efficiently throughout the entire casting volume as it solidifies.
How Feeding is Facilitated
To ensure proper feeding, casters often employ techniques such as:
- Risers: Reservoirs of liquid metal attached to the casting that remain liquid longer and feed metal back into the casting as it solidifies.
- Directional Solidification: Designing the casting and mold so that solidification progresses sequentially from the regions furthest from the risers towards the risers themselves, ensuring a continuous path for liquid metal flow.
- Chills: Metal inserts placed in the mold to locally increase the cooling rate, promoting directional solidification towards feeder areas.
In essence, "feed" in casting is the vital mechanism of supplying liquid metal to prevent shrinkage cavities during solidification, ensuring the casting's soundness and desired properties.
