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
nterested to talk with me about your plastic selection and part design needs - here you can contact me
Literature:
[1] Keuerleber and Eyerer:
Konstruieren und Gestalten mit Kuntstoffen, 2007
[2] https://www.findoutaboutplastics.com/2018/04/plastics-part-design-10-holy-design.html
[3] Designing with Plastics: A Practical Guide
for Engineers, DESIGN NEWS 11.20.06
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