Shrinkage in Injection Moulding: Causes, Material Behavior & Design Strategies

Shrinkage is one of the most important — and often misunderstood — characteristics of injection moulded plastics. As molten polymer cools inside a mould, it contracts. This dimensional change is unavoidable, yet highly predictable when understood properly.

Designing with shrinkage in mind allows engineers to achieve accurate parts, stable assemblies, and reliable production outcomes.

Ignoring it, however, is one of the fastest paths to tolerance failure.

What Causes Plastic Shrinkage?

When plastic transitions from a molten state to a solid, molecular chains pack closer together. This natural densification results in dimensional reduction.

Shrinkage occurs in two primary stages:

• Processing shrinkage: happens as the part cools inside the mould
• Post-mould shrinkage: minor dimensional change after ejection

Both must be considered during tool design.

Not All Plastics Behave the Same

Material structure plays a major role in shrinkage behavior.

Amorphous Plastics

These materials shrink less and more uniformly, making them ideal for precision applications.

Semi-Crystalline Plastics

Typically exhibit higher shrink rates due to tighter molecular packing.

Understanding this distinction is critical when specifying materials for tight tolerances.

Typical Shrinkage Ranges

• Low shrink materials: ~0.1% – 0.5%
• Moderate shrink: ~0.5% – 1.5%
• High shrink polymers: 1.5% – 3%+

Exact values vary by grade and processing conditions.

Key Factors That Influence Shrinkage

Material Selection

Polymer type is the single largest driver.

Wall Thickness

Thicker areas cool more slowly, increasing local shrink.

Cooling Rate

Uniform cooling promotes dimensional stability.

Part Geometry

Asymmetrical shapes can shrink unevenly, leading to warping.

Processing Conditions

Injection pressure, packing, and mould temperature all affect final dimensions.

How Toolmakers Compensate for Shrinkage

Experienced tool designers intentionally machine cavities larger than the desired final part.

This compensation ensures that once shrinkage occurs, the component settles within tolerance.

Accurate prediction is what separates stable production from repeated tooling adjustments.

Design Strategies That Improve Dimensional Stability

Maintain Uniform Wall Thickness

Promotes consistent cooling.

Avoid Heavy Sections

Core out thick regions wherever possible.

Design Symmetrical Geometry

Balanced shapes shrink more evenly.

Collaborate Early With Your Moulder

Manufacturing insight prevents unrealistic tolerance expectations.

Why Shrinkage Matters More at Scale

Minor dimensional variation might go unnoticed in prototype quantities — but becomes critical in high-volume production.

Predictable shrink behavior is essential for:

• Multi-part assemblies
• Snap fits
• Sealing surfaces
• Automated assembly