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 thickness of the casting. When the geometry of the molding cavity prevents directional solidification from occurring naturally, a chill can be strategically placed to help promote it. There are two types of chills: internal and external chills.
Internal chills are pieces of metal that are placed inside the molding cavity. When the cavity it filled, part of the chill will melt and ultimately become part of the casting, thus the chill must be the same material as the casting. Note that internal chills will absorb both heat capacity and heat of fusion energy.
External chills are masses of material that have a high heat capacity and thermal conductivity. They are placed on the edge of the molding cavity, and effectively become part of the wall of the molding cavity. This type of chill can be used to increase the feeding distance of a riser or reduce the number of risers required.
A chill that is commonly used in sand castings is made of iron, which has a higher density, thermal conductivity and thermal capacity than the mold material. Chills can be made of many materials, including iron, copper, bronze, aluminium, graphite, and silicon carbide. Other sand materials with higher densities, thermal conductivity or thermal capacity can also be used as a chill. For example, chromite sand or zircon sand can be used when molding with silica sand.
The process employs chill-casting techniques and a specialty-steel mold to produce precision castings made from copper and copper-base alloys.
Liquid metal enters the mold via gravity or low pressure. Pouring of the metal and movement of the mold are coordinated to avoid turbulence when the metal enters the mold.
Because molten metal flows into all parts of the mold by gravity pressure, each casting is dimensionally accurate and free of porosity. As the metal is poured, the mold is gently vibrated to ensure the complete filling necessary to produce sharp corners and fine detail in the castings.
After each casting cycle, the mold is water cooled to create a significant temperature difference between the liquid metal and the mold. This temperature difference causes a fine, dense grain structure.
The high-quality chill castings offer designers, engineers, and manufacturers many advantages including:
Engineered components with near-net-shape characteristics
-
Dimensional uniformity
- Improved mechanical properties
- Pressure tightness
- Close tolerances
- Dimensional accuracy
- Precision and dimensional repeatability
- Bright, smooth metallic surface.
