What is shrinkage in die casting?

Shrinkage is the reduction in volume that occurs when molten metal cools and solidifies inside a die casting mold. It is one of the most common defects and dimensional challenges in die casting.

Why It Happens

Molten metal contracts in three stages:

1. Liquid Shrinkage — Metal contracts while still in liquid state as it cools

2. Solidification Shrinkage — Volume reduces as metal transitions from liquid to solid

3. Solid Shrinkage — Further contraction as the solidified part cools to room temperature

Types of Shrinkage

1. Dimensional Shrinkage

• The finished part is slightly smaller than the mold cavity

• Accounted for by scaling the mold larger during design

• Typical shrinkage rates:

• Aluminium: 0.5–0.7%

• Zinc: 0.3–0.5%

• Magnesium: 0.5–0.8%

• Copper/Brass: 0.6–0.8%

2. Shrinkage Porosity

• Internal voids or cavities formed when outer shell solidifies before the core

• Trapped liquid metal contracts with nowhere to draw from

• Weakens mechanical strength and pressure tightness

• Hard to detect without X-ray or CT scanning

3. Surface Shrinkage (Sink Marks)

• Visible depressions or dimples on the part surface

• Common in thick sections, ribs, and bosses

• Caused by surface solidifying over a shrinking interior

4. Centerline Shrinkage

• Voids that form along the central axis of thick sections

• Result of simultaneous outer solidification trapping liquid in the middle

Factors That Influence Shrinkage

How To Minimise Shrinkage

Design Solutions

• Maintain uniform wall thickness throughout the part

• Add generous fillets to avoid abrupt section changes

• Avoid large isolated thick sections

• Follow the 3:1 rule — ribs should be no more than 3x the wall thickness

Process Solutions

• Optimise injection pressure and speed

• Control die and melt temperatures precisely

• Use proper gate sizing and placement to feed all sections

• Apply intensification pressure during solidification

• Ensure adequate venting to avoid back pressure

Tooling Solutions

• Design overflow wells to draw shrinkage away from critical areas

• Use chills or cooling inserts to control solidification direction

• Design feed system so metal solidifies progressively toward the gate

Shrinkage vs Porosity

Key Takeaway

Shrinkage is unavoidable in die casting but is manageable through good part design, proper tooling, and controlled process parameters. Accounting for shrinkage in the mold design stage is essential for achieving accurate final dimensions.