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Design Considerations for Injection Moulding: Ribs, Bosses & Undercuts

There are several design consideration to study whilst injection moulding some of which including Ribs, Bosses, and Undercuts. 




Ribs are thin support features (usually triangular shaped) on a part that are used for strengthening wall sections and bosses. Ribs are considered when designing to add strength into your injection moulding part. Ribs are also a more cost effective solution due to less material being required.


Ribs should be designed to be 50-60% of the wall thickness, thinner ribs do not provide as much structure and may be hard to fill. Thicker ribs are therefore more attractive, but can cause surface defects when injection moulding.




Bosses are the round protrusions on plastic parts and moulds, often designed for fasteners, they help strengthen parts and serve two purposes.


  1. Their primary use is to strengthen holes or slots within the component. Thin walls are ideal when designing for plastic injection moulding, but the challenge is ensuring these thin walls are also strong. Bosses help reinforce the component without impacting on cost or design.
  2. Their second use is to create points where the parts can fasten together at assembly. When designed correctly the bosses should line up and once the injection moulding parts are in place ready for a clip of fastening they should permanently fix them together.




Undercuts are a protrusion or indentation that impedes withdrawal from a two-piece rigid mould. When producing plastic parts with undercuts there’s a distinct challenge for plastic injection moulding designers. Undercuts are protrusions or recessions in a part that prevent its ejection from the mould. Undercut designs are often used to create threaded parts, for example screw top bottles.


If your injection moulding part requires undercuts, there are other design considerations that should be discussed and our designers will happily explain these to you. Get in touch with our team of experts today for help and advice when designing your part.




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Our team will be more than happy to help, advise and quote for your requirements.

Looking for more information? Check out our blog, News & Views for useful articles, tips and tricks on plastic injection moulding.