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Design Considerations for Injection Moulding: Ribs, Bosses & Undercuts

When designing for plastic injection moulding, there are several design considerations to study, this article shines a spotlight on ribs, bosses and undercuts. Ribs are thin support features (usually triangular shaped) on a part that are used for strengthening wall sections and bosses. Bosses are the round protrusions on plastic parts and moulds, often designed for fasteners. Undercuts are a protrusion or indentation that impedes withdrawal from a two-piece rigid mould.



It’s not uncommon for a small plastic injection molded part to be expected to hold up under a large load. When designing to add strength, ribs are often considered because they give the component stiffness and strength without increasing wall thickness (we discuss the significance of wall thickness in a different article, read it here). Because less material is required when using ribs (in comparison to increasing wall thickness) ribs can be a more cost-effective solution.

Unfortunately, ribs are not devoid of challenges and can increase the likelihood of warping or surface defects. However, by being conscious of part geometry, designers can ensure that adding ribs to the design will improve bending stiffness and part strength.  

Our design team have identified the following rules they would apply when working with ribs:

  • Rib thickness - 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 they can cause surface defects.
  • Rib height – The rib height should be under 3X the wall thickness for maximum effectiveness, to reduce stress, filling and ejection problems.
  • Rib spacing – Ribs should be spaced a minimum of 2X the wall thickness apart, to increase component stiffness increase the number of ribs.
  • Rib placement – Ribs should be positioned down the length of the area subject to bending for optimal support.
  • Rib draft – To ensure they release from the tooling during moulding, aim for a minimum 1 degree draft.
  • Rib coring – to help alleviate surface defects, core thick ribs from the back or below to create a blank area and thinner rib walls.
  • Rib radii – to increase rib strength, the radii should be a minimum of 0.5-1.0 of the wall thickness. We discuss radii in more detail, here.



Bosses are design elements that strengthen parts, they serve two purposes:

  1. Their primary use is to strengthen holes or slots within the component. As we’ve discussed previously, thin walls are ideal when designing for plastic injection moulding but the challenge is ensuring these thin walls are also strong. Using bosses will reinforce the component without impacting on cost or design.
  2. Their secondary use is to create points whereby the parts can be fastened together at assembly. When designed correctly the bosses should line up and should the parts in place ready for a clip or fastening to permanently fix them together.

Due to their nature, bosses will usually have to endure tension, compression or flexing therefore correct design and placement is key. Our design team have identified the following guidelines they would apply when working with bosses:

  • Boss thickness – Wall thickness for bosses should be between 50-60% of the wall thickness to minimise surface defects.
  • Boss height – We recommend that the height of a boss should be no more than 3X the outer diameter.
  • Boss spacing – The spacing between bosses should be at least 2X the wall thickness, to avoid quality or production issues.
  • Boss draft – The minimum draft on the outer diameter of the boss should be no less than 0.5 degrees. The minimum draft on the inner diameter should be no less that 0.25 degrees. These recommendations are to ensure easy ejection from the mold.
  • Boss Radius – The minimum radius of a boss should be between 25% and 50% of the wall thickness at the base of the boss.
  • Boss placement – To add extra strength and for better rigidity and material flow, bosses should be attached to the nearest sidewall.



Producing plastic parts with undercuts presents a distinct challenge for plastic injection designers and molders. Undercuts are protrusions or recessions in a part that prevent its ejection from the mold.

Undercut designs are often used to create threaded parts, for example screw-top bottles and cosmetic containers such as lipsticks. 

If your 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.


Get in touch today and talk to the specialists, click here.

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.