Plastic injection moulding or CNC milling: which manufacturing process is right for your part? The answer depends on volume, geometry, tolerances, and lead time. Both methods have clear strengths, but also situations in which they fall short. In this article, we compare the two processes so you can make an informed choice.

Volume is the first deciding factor

By Plastic injection moulding You invest in a mould beforehand. This investment ranges from a few thousand to tens of thousands of euros, depending on the complexity. After that, the price per part quickly decreases with volume. For large runs, injection moulding is structurally the cheapest option per part.

CNC milling has no upfront tooling investment. You pay per milling hour, regardless of volume. This makes milling attractive for low quantities, prototypes, and one-off production. However, at higher volumes, machine costs quickly become a bottleneck.

As a general rule: with a few hundred parts per year, the limit is often reached where injection moulding becomes financially more attractive. Eurotechniek calculates this precisely for each request, so you can make a well-founded choice based on your specific situation.

Material usage differs fundamentally

Injection moulding is an additive manufacturing process: you add material to a mould and there is virtually no waste. CNC milling is subtractive: you start with a block of material and mill away the excess. For expensive technical plastics such as PEEK, PPS or PA66 GF, this can result in a significant cost difference.

Injection moulding can process virtually all thermoplastic materials. From PP and ABS to PEEK and POM, the choice is wide. CNC milling also works with these materials, but requires the material to be available as a machinable block or sheet. Not every material is available in that form or is economically viable to mill.

A practical example: a housing made of glass fibre-reinforced PA66, which is used in large series, can be produced more cheaply and efficiently by injection moulding than by milling. Material usage is lower, cycle time is shorter and the unit price falls rapidly with increasing volumes.

Geometry and Design Specifications

CNC milling offers more freedom with complex external geometries. A milling cutter can reach surfaces, contours, and edges that are difficult or impossible to achieve with injection moulding without additional mould constructions. Parts with deep narrow grooves, sharp internal corners, or unusual joining surfaces are often easier to realise through milling.

Injection moulding imposes design requirements that must be considered at an early stage. Draft angles prevent the part from sticking in the mould. Uniform wall thickness prevents shrinkage and warping. Gate locations need to be in the right place to guide filling. Good preparation makes all the difference.

Eurotechniek carries out a DFM analysis as standard before a mould is built. We assess the design for manufacturability and propose adjustments that improve the quality of the final product. This way, we prevent costly changes at a later stage of the production process.

Tolerances and surface finish

CNC milling achieves tighter tolerances than injection moulding in its standard configuration. For precise fitting dimensions, bearing seats, or functional guides, milling offers greater certainty. Tolerances of plus or minus 0.01 millimetres are achievable under stable process conditions and with well-calibrated tooling.

Injection moulding generally works with tolerances of plus or minus 0.05 to 0.2 millimetres, depending on the material and mould quality. For most industrial applications, this is more than sufficient. For critical fit dimensions, post-processing may be necessary, which affects the cost comparison.

The surface quality of injection-moulded parts is directly dependent on the mould finish. A polished mould yields a smooth surface without post-processing. CNC milling produces a milled surface that requires further processing depending on the milling parameters. For functional applications, the milled surface is typically acceptable without an additional step.

Lead time and scalability

With CNC milling, you get started quickly. You submit a file, the milling machine runs, and within a few days you have a part. There is no mould-making process and no waiting time for tool production. This is a clear advantage for prototypes, urgent deliveries, and single items.

Injection moulding requires a ramp-up period. Producing a mould takes four to ten weeks depending on complexity. After delivery, a validation phase follows with T1 samples before mass production begins. This time is a real factor when you need parts quickly for a market launch or replacement.

But once the mould is in place, production scales up effortlessly. Cycle times of a few seconds per part are common. A run of ten thousand pieces will be delivered to you within a few working days. CNC milling does not scale up in this way, as each part is machined individually.

Use both methods at the appropriate time

The smartest approach is not to choose between injection moulding and CNC milling, but to use both at the point where they deliver the most. Use CNC milling in the development phase to create functional prototypes and validate the design. Switch to injection moulding once the design is stable and the volumes justify the mould investment.

This process requires coordination and foresight. Eurotechniek will guide you from the design phase through to mass production. We assess the prototype for itsmanufacturability for injection moulding and provide concrete recommendations that ensure a smooth transition. This way, you won't lose time owing to late design changes.

A concrete example: an industrial control panel is validated as a CNC-milled prototype by the end-user. Following approval, the design is adapted for injection moulding with integrated clip mechanisms and cable channels. From the second series run, the unit costs are twenty percent lower than the milled alternative.

Frequently asked questions about injection moulding or CNC milling

Can I use a milled prototype directly as the basis for an injection mould?

Not without an interim review. A CNC milled design does not take into account draft angles, gate locations and wall thickness requirements that are essential for injection moulding. Geometric details that are simple to mill can cause mould issues with injection moulding. Eurotechniek carries out a DFM analysis on the existing design and provides concrete adjustments before the mould is built, thus avoiding time- and money-consuming correction rounds.

From what volume is injection moulding cheaper than CNC milling?

That depends on the mould costs, the complexity of the part, and the milling time per piece. For simple parts and moulds costing between three and five thousand euros, the break-even point is often reached at five hundred to a thousand units per year. For more complex moulds with multiple slide mechanisms, this is higher. Eurotechniek performs a cost comparison for each request based on your volume and design, so you know precisely where the break-even point lies.

What tolerances can I achieve with injection moulding, and is post-processing sometimes necessary?

Injection moulding achieves standard tolerances of plus or minus 0.05 to 0.2 millimetres depending on the material and wall thickness. This is sufficient for most industrial applications. For critical mating dimensions or bearing seats, post-processing via milling or reaming may be necessary. Eurotechniek discusses this in advance during the design process so that tolerance requirements are incorporated into the mould design and any post-processing is scheduled as part of the series production.

The right method at the right time

Injection moulding and CNC milling are complementary processes. Milling offers you speed and precision for low volumes and complex geometries. Injection moulding offers you scalability, efficient material usage, and a low unit price for mass production. The choice is not a matter of preference, but of technical and economic logic.

Would you like to know which method is suitable for your component and production volume? Contact Eurotechniek via euro-techniek.nl and we'll discuss the best approach for your situation together.