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Injection moulding and CNC machining can both be used to manufacture accurate, repeatable plastic components, but they suit very different types of project.

If you already have a component drawing, the question is not simply whether the part can be moulded or machined. We also need to consider the quantity required, geometry, tolerances, material, expected production life and the amount of initial investment that makes sense for the project.

At Allthread, we manufacture custom injection-moulded plastic components but also have CNC machining capacity for parts where moulding is not the most appropriate option. Here are some of the main factors we would consider when deciding between the two.

Production Quantity

Quantity is often one of the first considerations.

Injection moulding requires tooling to be designed and manufactured before production can begin. This creates an initial cost that needs to be justified by the number of components being produced.

Once the tooling is in place, however, injection moulding allows the same component to be produced repeatedly and efficiently. For ongoing production requirements or larger quantities, the tooling cost can therefore be spread across the production run.

CNC machining works differently. Components are machined directly from solid plastic material, removing the need for an injection mould tool.

That can make CNC particularly useful for one-off components, prototypes, small batches or projects where the required quantity does not justify dedicated tooling.

Component Geometry

The shape of the component can also determine which process makes the most sense.

An injection-moulded component needs to be designed so that molten plastic can fill the mould correctly and the finished part can then be removed from the tool. Wall thickness, draft angles, undercuts and the position of features all need to be considered.

CNC machining does not have the same mould-release requirements.

Because material is removed mechanically from solid stock, machining can sometimes accommodate features that would significantly complicate an injection mould tool.

That does not automatically mean that a complex component should be machined. If thousands of identical parts are required, it may still be worthwhile adapting the design so that it can be moulded efficiently.

The intended production volume needs to be considered alongside the geometry.

Tolerances and Accuracy

Both processes can produce precise engineering components, but the way tolerances are achieved is different.

With CNC machining, dimensions are produced directly by the machining operation. This makes CNC particularly useful where a low-volume component requires tightly controlled dimensions.

Injection-moulded dimensions are influenced not only by the tooling but also by the behaviour of the plastic as it fills, cools and shrinks.

For moulded components with critical dimensions, the material, wall sections, tooling and tolerances therefore need to be considered together during the design stage.

Applying unnecessarily tight tolerances to every dimension can make either process more complicated and expensive, so it is useful to identify which dimensions are genuinely critical to the component’s function.

Development and Prototyping

CNC machining can also be valuable during development.

If an engineer wants to check the fit, basic dimensions or function of a component before committing to production tooling, machining a small number of parts may provide a practical way of evaluating the design.

Any differences between a machined prototype and the eventual moulded component still need to be understood, particularly where the manufacturing process affects the final part.

For some projects, machining is therefore the production method. For others, it is one stage in developing a component that will eventually be injection moulded.

Material Selection

Material should be considered at the same time as the manufacturing process.

At Allthread, we work with engineering plastics including Nylon 66, polypropylene, PVDF, acetal, polyethylene and polycarbonate.

The correct material will depend on factors such as mechanical loading, wear, chemical exposure, temperature, dimensional requirements and the environment in which the component will operate.

Material availability and how a particular plastic behaves during moulding or machining can also influence the process we recommend.

Which Process Should You Choose?

There is no fixed point at which CNC machining becomes wrong and injection moulding becomes right.

As a general guide, CNC machining is well suited to prototypes, one-off parts, small production quantities and components where dedicated mould tooling would be difficult to justify.

Injection moulding becomes increasingly attractive where a component is required repeatedly in larger quantities and can be designed effectively for the moulding process.

In some cases, we may suggest modifying a design to make injection moulding more practical. In others, machining the component may remain the more sensible option.

If you have a plastic component you need to manufacture, send us your drawing, CAD file or even an initial sketch. We can look at the component, quantities, material and application and advise whether injection moulding, CNC machining or another manufacturing method is likely to provide the most appropriate solution.