Shenzhen Alu Rapid Prototype Precision Co., Ltd.

Most metals shrink during casting due to two primary interconnected processes: solidification shrinkage and thermal contraction. 

Here's a breakdown of why this happens and its implications:

1. Solidification Shrinkage

When molten (liquid) metal is poured into a mold and begins to cool, it undergoes a phase change from liquid to solid.

For most metals (e.g., aluminum, steel, copper alloys), the solid phase is denser than the liquid phase. This means the atoms pack more tightly together upon solidification, resulting in a reduction in volume for the same mass of material.

Typical shrinkage rates during this phase range from 2-7% by volume, depending on the alloy (e.g., 2-3% for stainless steels and nickel alloys, up to 6-7% for aluminum). 

This is a fundamental property rooted in the atomic structure: liquids have more free volume between molecules, while solids form a more ordered, compact lattice.

2. Thermal Contraction

After solidification, the now-solid casting continues to cool from its freezing temperature (often 1,000–1,500°C or higher) down to room temperature.

Metals generally expand when heated and contract when cooled due to increased atomic vibrations at higher temperatures, which take up more space. As the temperature drops, these vibrations decrease, pulling atoms closer together.

This adds another 1-2% of shrinkage on top of the solidification phase, primarily occurring while the metal is still fully liquid (pre-solidification cooling) and then as a solid. 

Why "Most" Metals (Exceptions)

The phrase "most metals" accounts for rare exceptions where shrinkage is minimized or reversed. For instance:Gray cast iron expands slightly during solidification due to the formation of graphite flakes, which create internal voids that offset contraction.

Bismuth and a few other metals also expand on freezing, similar to water.

However, these are outliers; the vast majority of industrially cast metals (e.g., steels, bronzes, aluminums) exhibit net shrinkage. 

Shrinkage can lead to defects like voids, cracks, or porosity if not managed, as the outer surface solidifies first and constrains the inner material.

Foundry engineers compensate using techniques like risers (extra molten metal reservoirs), chills (to control cooling rates), or alloying elements to reduce shrinkage rates.