Rib Design for Injection Moulding: Engineering Guidelines for Stronger Plastic Parts

Ribs are among the most effective structural features available in injection moulding. When designed correctly, they dramatically improve stiffness and load distribution without requiring thicker walls — helping control both material usage and cycle time.

For engineers seeking to balance strength, weight, and cost, rib geometry often determines whether a part scales successfully into production.

Poor rib design, however, can introduce cosmetic defects, internal stress, and dimensional instability. Understanding the fundamentals is essential.

Why Use Ribs Instead of Thick Walls?

Increasing wall thickness may appear to strengthen a part, but it frequently creates manufacturing problems such as sink marks and extended cooling times.

Ribs provide a far more efficient solution.

• Improve stiffness without excessive material
• Promote faster cooling
• Reduce part weight
• Lower production cost
• Support dimensional stability

This is why ribs appear in everything from automotive housings to consumer electronics.

Recommended Rib Thickness

One of the most widely accepted design rules is maintaining rib thickness at roughly 50–60% of the adjoining wall.

This ratio helps prevent differential cooling — the primary cause of sink marks.

Example:

• Wall thickness: 3.0 mm
• Ideal rib thickness: ~1.5 – 1.8 mm

Exceeding this range increases cosmetic risk.

Optimal Rib Height

While taller ribs improve stiffness, excessively high ribs can become difficult to fill and eject.

A common guideline is:

• Maximum rib height: about 2.5–3× wall thickness

Beyond this, consider adding multiple ribs instead of one large feature.

Add Draft to Every Rib

Ribs must include draft angles to allow smooth ejection.

Typical recommendation:

• At least 0.5° draft per side
• More for deeper ribs

Skipping draft is a frequent cause of tooling revisions.

Use Generous Radii at the Base

Sharp transitions concentrate stress and restrict material flow.

A base radius of roughly 25–40% of wall thickness improves structural performance while supporting consistent filling.

Spacing Matters More Than Many Designers Realise

Placing ribs too close together can trap heat and create uneven shrink behaviour.

As a practical guideline:

• Maintain spacing of at least 2–3× wall thickness

This encourages uniform cooling.

Common Rib Design Mistakes

• Making ribs too thick
• Skipping draft angles
• Using sharp internal corners
• Designing ribs that are excessively tall
• Poor spacing that traps heat

These issues are far easier to prevent than to correct after tooling begins.

When Multiple Ribs Are Better Than One

Several thinner ribs typically outperform a single thick rib.

This approach distributes load more evenly and reduces cosmetic risk.

Collaborate Before Finalising Structural Features

Manufacturing input during the design stage helps confirm that ribs align with material behaviour and tooling strategy.

Early adjustments are inexpensive — late tooling changes are not.