Wednesday 27 October 2021

Rule of Thumb for Plastics Part Design – Ribs

 Hello and welcome to a new rule of thumb post. More rule of thumb posts can be found here.

In this post we discuss an important element in plastic part design: ribs. The design of proper ribs for increasing the stiffness of your part is one element of the 10 “holy” Design rules for injection moulded products.

Why are ribs so effective?

Ribs are just getting effective when they are 4-10 times higher than the wall thickness. The thickness of the rib should be 40% (minimize sink marks) - 60% (maximize strength) of the original wall. Furthermore, injection moulding location and filling direction (molecular orientation) affects how ribs will perform in a later stage. In general, it can be stated that if your ribs never exceed 40-60% of the nominal wall thickness and length of 4-10 times of nominal wall thickness, problems of sink are decreased and part stiffness is increased.

Rule of Thumb for Plastic Part Design: how to design ribs

When to consider ribs?

There are two main design routes for increasing the stiffness of your part: make a thick wall section or use ribs. The cross sectional moment of inertia I = b*h^3/ 12 shows that increasing the wall thickness will increase the stiffness of your part to the power of three. It is more effective than changing the material. However, cooling time will increase significantly too. Therefore, adding the rib may be the better solution. Adding a rib to a wall thickness of 8 mm will increase the part’s stiffness six times, compared to a wall thickness of 12 mm with no rib.  

Thanks for reading!

Greetings and #findoutaboutplastics

Herwig Juster

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[1] Keuerleber and Eyerer: Konstruieren und Gestalten mit Kuntstoffen, 2007


[3] Designing with Plastics: A Practical Guide for Engineers, DESIGN NEWS 11.20.06

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