Injection Moulding Tolerances: Design Guidelines for Precision Parts

Dimensional accuracy is critical in plastic component manufacturing — especially when parts must assemble seamlessly or interact with mechanical systems. Understanding achievable injection moulding tolerances helps engineers design products that function reliably without driving unnecessary production costs.

While modern tooling can deliver impressive precision, plastic behaves very differently from metal. Designing with realistic expectations is the key to avoiding expensive revisions later.

What Are Injection Moulding Tolerances?

Tolerance refers to the allowable variation in a part’s dimensions after manufacturing. Because thermoplastics shrink as they cool, achieving perfect nominal measurements is not possible — but predictable accuracy absolutely is.

Typical Tolerance Ranges

• General commercial reminder: ±0.2mm – ±0.5mm
• Precision applications: ±0.1mm – ±0.2mm
• Micro components: tighter tolerances may be achievable with specialised tooling

The tighter the tolerance, the more control is required across tooling, materials, and processing conditions.

What Influences Dimensional Accuracy?

Material Shrinkage

Every polymer shrinks at a different rate. Semi-crystalline plastics typically shrink more than amorphous materials, which directly affects achievable precision.

Tool Design

A well-engineered mould compensates for shrinkage before the first part is ever produced. Poor tool design is one of the fastest paths to tolerance failure.

Part Geometry

Uniform wall thickness promotes even cooling — a major factor in dimensional stability.

Features that increase risk include:

• Thick sections
• Sharp transitions
• Asymmetrical designs
• Large flat surfaces

Processing Conditions

Temperature, injection pressure, and cooling time must remain tightly controlled. Even small fluctuations can shift final dimensions.

When Tight Tolerances Make Sense — And When They Don’t

Specifying extremely tight tolerances across an entire part is rarely necessary.

Instead, prioritise precision only where function demands it:

• Mating features
• Snap fits
• Bearing surfaces
• Sealing interfaces

Relaxing non-critical dimensions often reduces tooling complexity and inspection costs.

Design Strategies for Better Accuracy

Keep Walls Consistent

Uniform thickness supports predictable shrink behaviour.

Add Generous Radii

Smooth transitions reduce internal stress.

Avoid Overly Large Parts

Larger components experience greater thermal variation during cooling.

Collaborate Early With Your Moulder

Manufacturing input during the design phase prevents unrealistic tolerancing.