Authored By: SDI Plastics

When it comes to producing high-quality plastic components efficiently, nothing beats the precision of plastic injection moulding. But if you’re aiming for consistency, cost-effectiveness, and performance from your moulded parts, everything starts with the plastic design for injection moulding. It’s not just about melting plastic and shaping it, it’s about engineering with intent.

Whether you’re designing a new consumer product, automotive part, or industrial component, the design stage has the biggest impact on final output. 

Let’s walk through some of the best practices to follow when designing for plastic injection moulding, and why getting the design right from the beginning can save you serious time, money, and headaches later on.

1. Understand material behaviour before designing

Different plastics behave differently under stress, heat, and pressure. A polypropylene part will react differently than one made from ABS or nylon. Before you even draw the first sketch, consider what the part will be exposed to, UV light, chemicals, temperature extremes, or mechanical wear?

The trick here is aligning your plastic design for injection moulding with the characteristics of the chosen material. Don’t design assuming all plastics behave the same; you’ll end up with warped, brittle, or inconsistent parts.

2. Wall thickness consistency is key

One of the golden rules in plastic design for injection moulding is to keep wall thickness consistent wherever possible. Sudden changes in thickness can cause warping, sink marks, and internal stress due to uneven cooling rates.

As a rule of thumb, aim for a wall thickness of 1.5mm to 3mm for most parts, but this will vary based on the type of plastic. If you do need to change thickness, taper it gradually. Avoid large jumps in thickness, it’s a recipe for defects.

3. Incorporate draft angles in your design

Here’s something many new designers overlook: parts need to be released from the mould easily. If the vertical walls of your part are straight (i.e., no draft angle), they’ll grip the mould and get stuck, which can damage the part, or the mould.

Adding a slight draft angle (typically 1° to 2° per side) helps the part release cleanly without extra force or tooling. It’s a small adjustment in the design stage that saves big in the production line.

4. Use ribs and gussets instead of solid mass

Want strength without bulk? Avoid thick, chunky sections and go for ribs or gussets instead. These features add rigidity without increasing material usage (and cycle time).

Ribs should be around 60% of the wall thickness to prevent sink marks, and spaced properly to avoid stress concentration. It’s all about smart support rather than just bulking things up.

5. Design with the moulding process in mind

Injection moulding isn’t just about shaping a part, it’s a thermal and mechanical process. Gates, runners, venting, and ejection must all be considered.

A successful plastic design for injection moulding always anticipates where the plastic will flow, where it will cool, and how it will be ejected. For example, placing gates near the thickest area of the part helps with proper filling and minimises voids or sink. Likewise, adding ejector pin support areas in your design ensures smooth removal from the tool.

6. Avoid sharp corners

Sharp internal corners create stress risers in plastic parts. These are weak points where cracking can begin under load or over time. Use fillets instead, rounded transitions not only improve strength but also allow plastic to flow more evenly through the mould.

Rounded corners = stronger, more durable parts. It’s that simple.

Final thoughts

In plastic injection moulding, success truly lies in the design. When your plastic design for injection moulding is optimised, everything else becomes easier, tooling, manufacturing, quality control, and long-term performance.

Design with manufacturability in mind. Collaborate early with engineers and toolmakers. And always prototype before committ