How to Design a Part for Injection Moulding

Designing for injection moulding is all about balance – getting the look and function you want, while also making sure your part can be moulded efficiently and reliably.

Here are some key injection moulding design tips to help you get started.

Keep Wall Thickness Consistent

Thick sections cool more slowly than thin ones, which can lead to warping, sink marks and internal stresses. As a rule, aim for consistent wall thickness throughout your design.

Where you do need thicker features – such as bosses or ribs – try to hollow out the part or “core” the area. This reduces the amount of material used and helps with cooling.

Avoid Sharp Corners

Sharp corners can cause stress concentrations, reduce material flow, and lead to weak points. Wherever possible, use radii or fillets to smooth transitions between faces.

Fillets also improve flow during moulding and reduce wear on the tool.

Watch Out for Undercuts

Undercuts are features that prevent the part from being ejected straight out of the tool. These usually require sliding cores or lifters, which add cost and complexity.

Try to design features that open in the direction of tool movement. If you do need an undercut, we’ll help you find the simplest solution.

Add Ribs for Strength

Ribs are a great way to reinforce parts without increasing wall thickness. Just make sure they’re not too thick – ideally, they should be around 60% of the adjoining wall to avoid sink marks.

Think About the Injection Point

Where the molten plastic enters the cavity – known as the gate – can affect the flow, cooling and final appearance of the part. It’s best to position the gate where there’s enough wall thickness to handle the flow and pressure.

Feeding into a thin section can lead to short shots or weak weld lines.

Choose the Right Material

Different polymers behave differently in the mould. Some are prone to warping, others to gassing or sticking. The material you choose will affect everything from draft angles to venting and ejection.

If you’re unsure, we can advise on the best options based on your part geometry, required strength and end use.

Need support with your moulding design?

Whether you’re refining an early-stage concept or optimising a final design, our team can help you avoid common pitfalls. Talk to us today about your project.