Designing smarter features for stronger, more efficient parts
When it comes to designing parts for injection moulding, small details make a big difference. Features such as ribs, bosses, and undercuts all play an important role in the strength, functionality, and manufacturability of a component. Getting these right in the design stage can prevent costly tooling changes, improve performance, and ensure efficient production.
At CJ Tool & Mouldings, we support design engineers and product developers to refine these critical features, ensuring that every part is optimised for both performance and production.
Ribs – Adding Strength Without Excess Material
Ribs are one of the most effective ways to reinforce a moulded part without increasing wall thickness. By strategically adding ribs, designers can boost stiffness and reduce warping, while keeping parts lightweight and cost-efficient.
Design considerations for ribs:
- Thickness: Ribs should typically be 50–60% of the adjoining wall thickness to avoid sink marks on the visible surface.
- Height: Excessively tall ribs can cause filling issues. A good rule of thumb is a maximum rib height of 3 x wall thickness.
- Draft: Adding a draft angle (0.5°–1°) to ribs helps ensure clean ejection from the mould.
- Spacing: Adequate spacing between ribs reduces the risk of thick sections and uneven cooling.
Bosses – Designing for Assembly
Bosses are often used to provide fastening points for screws, inserts, or alignment pins. Poorly designed bosses can lead to cracking, warping, or difficulties during assembly, so careful consideration is essential.
Design considerations for bosses:
- Wall Thickness: The outer wall of a boss should be no more than 60% of the adjoining wall thickness to prevent sink marks.
- Support: Use gussets or ribs to support tall bosses, reducing the risk of stress or breakage.
- Draft Angles: Apply at least 0.5° draft to boss walls for easier ejection.
- Location: Where possible, place bosses away from corners and parting lines to reduce stress.
Undercuts – Managing Complex Geometry
Undercuts are features that prevent a part from being ejected in a straight pull from the mould. They can be internal (like snap-fits or holes perpendicular to the moulding direction) or external (like hooks or grooves). While undercuts allow for clever part design, they add complexity and cost to tooling.
Design considerations for undercuts:
- Minimise if possible: Where a design can be achieved without undercuts, it is usually more efficient.
- Tooling Solutions: If unavoidable, undercuts may be moulded using slides, lifters, or collapsible cores.
- Part Function vs. Cost: Always weigh the functional benefit of an undercut against the additional tooling complexity it introduces.
- Tolerance & Durability: Complex mechanisms for undercuts require precise engineering and can increase tool maintenance needs.
Balancing Strength, Functionality, and Cost
Ribs, bosses, and undercuts are essential design features, but they must be carefully engineered to ensure manufacturability, performance, and cost-effectiveness. Over-designed parts can slow down production and raise tooling costs, while under-engineered parts may fail in service.
That’s where our expertise comes in. At CJ Tool & Mouldings, our team works closely with customers during the design stage, combining experience, mould flow analysis, and in-house tooling knowledge to deliver parts that are right first time.
From concept to component, we’re your trusted partner for injection moulded part design.
If you’d like expert advice on incorporating ribs, bosses, or undercuts into your next project, get in touch with CJ Tool & Mouldings today.
