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A well-designed injection-moulded component needs to work in two ways: it must perform correctly in its application, and it must be suitable for the moulding process itself.

A part that looks straightforward on a CAD drawing can become difficult or expensive to manufacture if wall thickness, draft, material choice, tolerances and tool release have not been considered early enough.

At Allthread, we manufacture custom plastic components in the UK and can support from an initial sketch or drawing through to finished production. Here are some of the main factors to consider when designing a plastic part for injection moulding.

Keep Wall Thickness Consistent

Where possible, wall thickness should remain reasonably consistent throughout the component.

Large variations in thickness can cause different areas of the moulding to cool at different rates, increasing the risk of sink marks, warping, internal stress and dimensional variation.

Making a wall thicker also does not necessarily make the component stronger. If additional stiffness is required, ribs or gussets can often reinforce the part without creating unnecessarily heavy sections.

The correct wall thickness will depend on the material, geometry and mechanical requirements of the component.

Allow for Draft

Once the plastic has cooled, the moulded component needs to be removed from the tooling. Draft is a slight taper applied to surfaces running in the direction of ejection. Without sufficient draft, the component may grip the mould as it cools, making it more difficult to release and potentially causing marking or distortion.

The amount of draft required will depend on the material, depth of the feature, surface finish and mould design.

It is therefore much better to consider draft during the initial design process rather than trying to add it once the component geometry has already been finalised.

Avoid Unnecessary Sharp Corners

Sharp internal corners can create concentrated areas of stress within a plastic component.

Where the application allows, adding an appropriate radius helps distribute that stress more gradually and can also improve the flow of molten plastic through the mould.

This does not mean every edge needs to be rounded. The aim is to avoid sharp internal transitions where they provide no functional advantage.

Use Ribs and Gussets Effectively

Ribs and gussets can be used to strengthen a moulded component without substantially increasing its wall thickness or weight.

Ribs are commonly used to stiffen larger surfaces, while gussets can reinforce projections, bosses and other loaded areas.

These features still need to be proportioned correctly. An excessively thick rib can create a localised heavy section and introduce the same cooling and shrinkage problems that the design is intended to avoid.

Consider Undercuts and Tool Release

An undercut is any feature that prevents the component from being removed directly from a conventional two-part mould.

Grooves, hooks, side holes and certain projections can all create undercuts.

They do not necessarily prevent a component from being injection moulded, but additional tooling mechanisms may be required. This can increase tool complexity and cost.

If an undercut is essential to the function of the part, it can be accommodated. If it is not essential, simplifying the geometry may make the component easier and more economical to manufacture.

Choose the Material Early

Material selection should be considered alongside the component design rather than after it.

Allthread works with engineering plastics including Nylon 66, polypropylene, PVDF, acetal, polyethylene and polycarbonate. Each behaves differently depending on load, temperature, moisture, chemical exposure, wear and dimensional requirements.

Different plastics also behave differently during moulding and cooling, so material choice can influence both component geometry and tooling.

Specify Tolerances Where They Matter

Not every dimension needs the same level of accuracy.

Tight tolerances may be essential for mating surfaces, fixing points or critical assembly dimensions, but applying unnecessarily tight tolerances across an entire component can make manufacturing more difficult and expensive.

Plastic components also behave differently from machined metal parts. Shrinkage, temperature, moisture and geometry can all affect dimensional stability.

Engineering drawings should therefore identify the dimensions that are genuinely critical to the function of the finished component.

What Should You Send an Injection Moulding Manufacturer?

When discussing a new component, it helps to provide as much information as possible, including:

• A CAD file, drawing or sketch;
• Overall dimensions and critical tolerances
• Expected loads;
• Operating temperatures;
• Exposure to moisture or chemicals;
• Preferred material, where known;
• And anticipated production quantities.

You do not need to have every detail resolved before getting in touch.

At Allthread, our design team can work from an initial concept and help develop the component, select a suitable material and determine whether injection moulding, CNC machining or plate cutting provides the most appropriate manufacturing route.