Draft Angles in Injection Moulding: Design Guidelines for Smooth Part Ejection

Draft angles are one of the most fundamental principles in injection moulding design. By introducing a slight taper to vertical walls, engineers ensure that parts release cleanly from the mould without damaging surfaces or placing excessive stress on tooling.

While draft may appear to be a minor geometric detail, insufficient taper is a frequent cause of cosmetic defects, tooling wear, and unexpected production delays.

Designing with proper draft from the outset is one of the simplest ways to improve manufacturability.

What Is a Draft Angle?

A draft angle is the degree of taper applied to walls parallel to the direction of mould opening.

As plastic cools, it shrinks slightly and grips the core of the mould. Draft reduces friction during ejection, allowing the part to separate smoothly.

Without it, components may drag against the tool surface — risking scratches, deformation, or even part failure.

Why Draft Angles Matter

• Protect part surface quality
• Reduce ejection force
• Extend tool lifespan
• Improve cycle consistency
• Lower risk of production interruptions

In high-volume manufacturing, these advantages compound quickly.

Recommended Draft Angles

Exact requirements vary depending on material, depth, and surface finish, but several widely accepted guidelines exist.

• Absolute minimum: ~0.5°
• Standard design target: 1°–2°
• Deep features: may require additional taper
• Textured surfaces: often need 3° or more

When uncertainty exists, adding slightly more draft is almost always safer than adding too little.

The Impact of Surface Finish

Surface texture dramatically increases friction during ejection.

As a general rule:

• Light textures → increase draft modestly
• Medium textures → require additional taper
• Deep textures → may demand aggressive draft angles

Ignoring this relationship is a common source of tooling revisions.

What Happens When Draft Is Insufficient?

Parts without adequate taper frequently stick inside the mould.

This can lead to:

• Drag marks
• Stress whitening
• Distortion
• Cracked features
• Premature tool wear

Correcting draft after tooling begins is significantly more expensive than addressing it during design.

Design Best Practices

Add Draft Early

Incorporating taper during CAD development avoids later compromises.

Align Draft With Shut-Off Directions

Ensure draft follows the natural opening direction of the mould.

Consider Material Behaviour

Softer polymers may tolerate slightly less draft, while rigid materials often benefit from more generous taper.

Collaborate Before Tooling

Manufacturing input helps confirm that draft aligns with the planned mould architecture.