In casting, a chill is a tool specifically used to control how metal cools and solidifies within a mold.
Understanding Chills in Metal Casting
A chill is an object used to promote solidification in a specific portion of a metal casting mold. Normally, the metal in the mold cools at a certain rate relative to the thickness of the casting. Chills are strategically placed to accelerate this cooling process in targeted areas.
Why Use Chills?
The primary purpose of using chills is to ensure that the casting solidifies progressively from thinner sections towards thicker sections, or towards dedicated feedmetal reservoirs (like risers). This controlled solidification helps prevent common casting defects.
Key reasons for using chills include:
- Preventing Shrinkage Cavities: As metal cools and solidifies, it shrinks. If thicker sections cool slower than thinner ones, shrinkage can create voids or cavities within the casting. Chills speed up solidification in these critical areas, ensuring they solidify soundly before surrounding metal shrinks away.
- Refining Grain Structure: Rapid cooling induced by chills can result in a finer grain structure in the metal, potentially improving the mechanical properties of the casting in that specific area.
- Achieving Desired Mechanical Properties: By controlling the cooling rate, chills can help achieve specific hardness or strength characteristics in localized areas of the part.
How Chills Work
Chills are typically made of materials with high thermal conductivity, such as iron, steel, or copper. When molten metal comes into contact with a chill, the chill quickly absorbs heat away from the metal. This accelerated heat transfer causes the metal directly contacting the chill to cool and solidify much faster than the surrounding metal that is cooling only through the mold material.
Types of Chills
Chills can be broadly categorized based on their placement:
- External Chills: These are placed against the outer surface of the mold cavity. They draw heat through the mold wall and are often larger.
- Internal Chills: These are placed directly within the mold cavity, becoming encapsulated by the molten metal during pouring. They must be made of a material compatible with the casting alloy to avoid contamination, and they melt or fuse into the casting.
Practical Application
Consider a casting design with a thick section adjacent to a thinner section. Without a chill, the thick section would solidify last, potentially developing a shrinkage cavity. Placing a chill on the surface or within the mold adjacent to the thick section forces that area to cool faster, promoting directional solidification towards a riser or another part of the casting designed to accommodate shrinkage.
In summary, chills are essential tools in foundry practice, used to manipulate the natural cooling rate of molten metal in specific areas of a mold to improve casting quality and integrity.
